Semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device with a structure in which an increase in the number of oxygen vacancies in an oxide semiconductor layer can be suppressed and a method for manufacturing the semiconductor device are provided. The semiconductor device includes an oxide insulating layer; intermediate layers apart from each other over the oxide insulating layer; a source electrode layer and a drain electrode layer over the intermediate layers; an oxide semiconductor layer that is electrically connected to the source electrode layer and the drain electrode layer and is in contact with the oxide insulating layer; a gate insulating film over the source electrode layer, the drain electrode layer, and the oxide semiconductor layer; and a gate electrode layer that is over the gate insulating film and overlaps with the source electrode layer, the drain electrode layer, and the oxide semiconductor layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, thepresent invention relates to, for example, a semiconductor device, adisplay device, a light-emitting device, a power storage device, animaging device, a driving method thereof, or a manufacturing methodthereof. One embodiment of the present invention relates to asemiconductor device or a method of manufacturing the semiconductordevice.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

A technique by which a transistor is formed using a semiconductor filmformed over a substrate having an insulating surface has been attractingattention. The transistor is used in a wide range of electronic devicessuch as an integrated circuit (IC) and an image display device (displaydevice). A silicon-based semiconductor material is widely known as amaterial for a semiconductor thin film that can be used in a transistor.As another material, an oxide semiconductor has been attractingattention.

For example, a transistor whose active layer includes an amorphous oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn) isdisclosed in Patent Document 1.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

SUMMARY OF THE INVENTION

It is known that an oxygen vacancy in an oxide semiconductor contributesto the formation of a donor; thus, in the case where the oxidesemiconductor is used for a channel formation region of a transistor, anoxide semiconductor layer including as few oxygen vacancies as possibleis preferably used.

However, even when an oxide semiconductor layer includes few oxygenvacancies initially, oxygen vacancies will increase in number fromvarious causes in a manufacturing process of a transistor. An increasein the number of oxygen vacancies in an oxide semiconductor layer causespoor electrical characteristics in some cases; for example, thetransistor becomes normally-on, leakage current increases, or thresholdvoltage is shifted due to stress application.

Therefore, an object of one embodiment of the present invention is toprovide a semiconductor device with a structure in which an increase inthe number of oxygen vacancies in an oxide semiconductor layer can besuppressed. Another object is to provide a semiconductor device with astructure in which the number of oxygen vacancies in an oxidesemiconductor layer can be reduced. Another object is to provide asemiconductor device with favorable electrical characteristics. Anotherobject is to provide a highly reliable semiconductor device. Anotherobject is to provide a semiconductor device with low power consumption.Another object is to provide a semiconductor device that can bemanufactured in a simple process. Another object is to provide a novelsemiconductor device or the like. Another object is to provide amanufacturing method of the semiconductor device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention relates to a transistorincluding an oxide semiconductor layer in a channel formation region andincluding a source electrode layer and a drain electrode layer incontact with a surface of the oxide semiconductor layer that is oppositeto a surface on which a gate electrode layer is formed with a gateinsulating film provided therebetween.

One embodiment of the present invention is a semiconductor deviceincluding an oxide insulating layer; intermediate layers apart from eachother over the oxide insulating layer; a source electrode layer and adrain electrode layer over the intermediate layers; an oxidesemiconductor layer that is electrically connected to the sourceelectrode layer and the drain electrode layer and is in contact with theoxide insulating layer; a gate insulating film over the source electrodelayer, the drain electrode layer, and the oxide semiconductor layer; anda gate electrode layer that is over the gate insulating film andoverlaps with the source electrode layer, the drain electrode layer, andthe oxide semiconductor layer.

The intermediate layer is preferably formed using a single layer orstacked layers of one or more materials selected from aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, yttria-stabilizedzirconia (YSZ), titanium nitride, tantalum nitride, gold, platinum,palladium, and ruthenium.

Top surface areas of the intermediate layers can be substantially thesame as top surface areas of the source electrode layer and the drainelectrode layer or can be larger than the top surface areas of thesource electrode layer and the drain electrode layer.

Top surface shapes of the gate electrode layer, the gate insulatingfilm, and the oxide semiconductor layer can be substantially the same orsimilar to each other.

For the oxide semiconductor layer, an In-M-Zn oxide (M is Al, Ti, Ga, Y,Zr, La, Ce, Nd, or Hf) or the like can be used.

Furthermore, the oxide semiconductor layer preferably includes crystalswith c-axis alignment.

The oxide semiconductor layer can be a stack in which a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer are formed in this order from the oxideinsulating layer side.

The oxide semiconductor layer may include a stack in which the firstoxide semiconductor layer and the second oxide semiconductor layer areformed in this order from the oxide insulating layer side and the thirdoxide semiconductor layer covering part of the stack.

A conduction band minimum of the first oxide semiconductor layer and aconduction band minimum of the third oxide semiconductor layer arepreferably closer to a vacuum level than that of the second oxidesemiconductor layer by 0.05 eV or more and 2 eV or less.

It is preferable that the first oxide semiconductor layer, the secondoxide semiconductor layer, and the third oxide semiconductor layer eachinclude an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf),and that an atomic ratio of M to In in each of the first oxidesemiconductor layer and the third oxide semiconductor layer be largerthan an atomic ratio of M to In in the second oxide semiconductor layer.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of forming anoxide insulating layer over a substrate, forming a thin film over theoxide insulating layer, forming a first conductive film over the thinfilm, forming a stack of an intermediate layer and a source electrodelayer and a stack of an intermediate layer and a drain electrode layerby selectively etching the thin film and the first conductive film witha first resist mask, forming an oxide semiconductor film over thestacks, forming an oxide semiconductor layer by selectively etching theoxide semiconductor film with a second resist mask, forming a gateinsulating film over the stacks and the oxide semiconductor layer,forming a second conductive film over the gate insulating film, andforming a gate electrode layer by selectively etching the secondconductive film with a third resist mask.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of forming anoxide insulating layer over a substrate, forming a thin film over theoxide insulating layer, forming a first conductive film over the thinfilm, forming a stack of an intermediate layer and a source electrodelayer and a stack of an intermediate layer and a drain electrode layerby selectively etching the thin film and the first conductive film witha first resist mask, forming an oxide semiconductor film over thestacks, forming a gate insulating film over the oxide semiconductorfilm, forming a second conductive film over the gate insulating film,and forming a gate electrode layer and an oxide semiconductor layer byselectively etching the second conductive film, the gate insulatingfilm, and the oxide semiconductor film with a second resist mask.

For the oxide semiconductor layer, an In-M-Zn oxide (M is Al, Ti, Ga, Y,Zr, La, Ce, Nd, or Hf) or the like can be used.

Furthermore, the oxide semiconductor layer preferably includes crystalswith c-axis alignment.

The oxide semiconductor layer can be a stack in which a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer are formed in this order from the oxideinsulating layer side.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of forming anoxide insulating layer over a substrate, forming a thin film over theoxide insulating layer, forming a first conductive film over the thinfilm, forming a stack of an intermediate layer and a source electrodelayer and a stack of an intermediate layer and a drain electrode layerby selectively etching the thin film and the first conductive film witha first resist mask, forming a first oxide semiconductor film and asecond oxide semiconductor film over the stacks, forming a first oxidesemiconductor layer and a second oxide semiconductor layer byselectively etching the first oxide semiconductor film and the secondoxide semiconductor film with a second resist mask, forming a thirdoxide semiconductor film over the stacks, the first oxide semiconductorlayer, and the second oxide semiconductor layer, forming a gateinsulating film over the third oxide semiconductor film, forming asecond conductive film over the gate insulating film, and forming a gateelectrode layer and a third oxide semiconductor layer by selectivelyetching the second conductive film, the gate insulating film, and thethird oxide semiconductor film with a third resist mask.

Furthermore, the first oxide semiconductor layer, the second oxidesemiconductor layer, and the third oxide semiconductor layer preferablyinclude crystals with c-axis alignment.

The thin film is preferably formed using a single layer or stackedlayers of one or more materials selected from aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, hafnium oxynitride, yttria-stabilizedzirconia (YSZ), titanium nitride, tantalum nitride, gold, platinum,palladium, and ruthenium.

Furthermore, a conduction band minimum of the first oxide semiconductorlayer and a conduction band minimum of the third oxide semiconductorlayer are preferably closer to a vacuum level than that of the secondoxide semiconductor layer by 0.05 eV or more and 2 eV or less.

It is preferable that the first oxide semiconductor layer, the secondoxide semiconductor layer, and the third oxide semiconductor layer eachinclude an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf),and that an atomic ratio of M to In in each of the first oxidesemiconductor layer and the third oxide semiconductor layer be largerthan an atomic ratio of M to In in the second oxide semiconductor layer.

According to one embodiment of the present invention, a semiconductordevice with a structure in which an increase in the number of oxygenvacancies in an oxide semiconductor layer can be suppressed can beprovided. A semiconductor device with a structure in which the number ofoxygen vacancies in an oxide semiconductor layer can be reduced can beprovided. A semiconductor device with favorable electricalcharacteristics can be provided. A highly reliable semiconductor devicecan be provided. A semiconductor device with low power consumption canbe provided. A semiconductor device that can be manufactured in a simpleprocess can be provided. A novel semiconductor device or the like can beprovided. A manufacturing method of the semiconductor device can beprovided.

Note that the description of these effects does not disturb theexistence of other effects. In one embodiment of the present invention,there is no need to obtain all the above effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a top view and cross-sectional views illustrating atransistor;

FIG. 2A is a top view of a transistor and FIG. 2B illustrates structuresof samples for experiment;

FIG. 3 shows results of TDS measurement;

FIGS. 4A to 4C are a top view and cross-sectional views illustrating atransistor;

FIGS. 5A to 5C are a top view and cross-sectional views illustrating atransistor;

FIGS. 6A and 6B are cross-sectional views illustrating a transistor;

FIGS. 7A to 7C illustrate a method for manufacturing a transistor;

FIGS. 8A to 8C illustrate a method for manufacturing a transistor;

FIGS. 9A to 9C illustrate a method for manufacturing a transistor;

FIGS. 10A to 10C illustrate a method for manufacturing a transistor;

FIGS. 11A to 11C are a top view and cross-sectional views illustrating atransistor;

FIG. 12 is a cross-sectional view illustrating a transistor;

FIGS. 13A to 13C illustrate a method for manufacturing a transistor;

FIGS. 14A to 14C are cross-sectional TEM images and a local Fouriertransform image of an oxide semiconductor;

FIGS. 15A and 15B show nanobeam electron diffraction patterns of oxidesemiconductor films and FIGS. 15C and 15D illustrate an example of atransmission electron diffraction measurement apparatus;

FIG. 16A shows an example of structural analysis by transmissionelectron diffraction measurement and FIGS. 16B and 16C show plan-viewTEM images;

FIGS. 17A and 17D are cross-sectional views of semiconductor devices andFIGS. 17B and 17C are circuit diagrams of semiconductor devices;

FIG. 18 is a cross-sectional view of a semiconductor device;

FIGS. 19A and 19C are circuit diagrams of memory devices and FIG. 19B isa cross-sectional view of a memory device;

FIG. 20 is a cross-sectional view of a semiconductor device;

FIG. 21 illustrates a configuration example of an RF tag;

FIG. 22 illustrates a configuration example of a CPU;

FIG. 23 is a circuit diagram of a memory element;

FIG. 24A illustrates a structure example of a display device, and FIGS.24B and 24C are circuit diagrams of pixels;

FIG. 25 illustrates a display module;

FIGS. 26A to 26F illustrate electronic devices; and

FIGS. 27A to 27F illustrate usage examples of an RF tag.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the following descriptionand it will be readily appreciated by those skilled in the art thatmodes and details can be modified in various ways without departing fromthe spirit and the scope of the present invention. Therefore, thepresent invention should not be construed as being limited to thedescription of the embodiments below. Note that in the structures of thepresent invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and the description thereof is not repeated in somecases. It is also to be noted that the same components are denoted bydifferent hatching patterns in different drawings, or the hatchingpatterns are omitted in some cases.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, without limiting to a predetermined connection relation,for example, a connection relation shown in drawings or texts, anotherconnection relation is included in the drawings or the texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

For example, in the case where X and Y are directly connected, X and Yare connected without an element that enables electrical connectionbetween X and Y (e.g., a switch, a transistor, a capacitor, an inductor,a resistor, a diode, a display element, a light-emitting element, or aload) interposed between X and Y.

For example, in the case where X and Y are electrically connected, oneor more elements that enable an electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off. That is, the switch is conducting or not conducting(is turned on or off) to determine whether current flows therethrough ornot. Alternatively, the switch has a function of selecting and changinga current path. Note that the case where X and Y are electricallyconnected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a D/A converter circuit, anA/D converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, and a buffer circuit; a signal generation circuit; amemory circuit; or a control circuit) can be connected between X and Y.For example, even when another circuit is interposed between X and Y, Xand Y are functionally connected if a signal output from X istransmitted to Y. Note that the case where X and Y are functionallyconnected includes the case where X and Y are directly connected and Xand Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are connected” means that X and Y are electrically connected(i.e., the case where X and Y are connected with another element orcircuit provided therebetween), X and Y are functionally connected(i.e., the case where X and Y are functionally connected with anothercircuit provided therebetween), and X and Y are directly connected(i.e., the case where X and Y are connected without another element orcircuit provided therebetween). That is, in this specification and thelike, the explicit expression “X and Y are electrically connected” isthe same as the explicit simple expression “X and Y are connected”.

For example, any of the following expressions can be used for the casewhere a source (or a first terminal or the like) of a transistor iselectrically connected to X through (or not through) Z1 and a drain (ora second terminal or the like) of the transistor is electricallyconnected to Y through (or not through) Z2, or the case where a source(or a first terminal or the like) of a transistor is directly connectedto one part of Z1 and another part of Z1 is directly connected to Xwhile a drain (or a second terminal or the like) of the transistor isdirectly connected to one part of Z2 and another part of Z2 is directlyconnected to Y.

Examples of the expressions include, “X Y, a source (or a first terminalor the like) of a transistor, and a drain (or a second terminal or thelike) of the transistor are electrically connected to each other, and Xthe source (or the first terminal or the like) of the transistor, thedrain (or the second terminal or the like) of the transistor, and Y areelectrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit structure is defined byan expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include “a source (or a first terminalor the like) of a transistor is electrically connected to X through atleast a first connection path, the first connection path does notinclude a second connection path, the second connection path is a pathbetween the source (or the first terminal or the like) of the transistorand a drain (or a second terminal or the like) of the transistor, Z1 ison the first connection path, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leasta third connection path, the third connection path does not include thesecond connection path, and Z2 is on the third connection path”, “asource (or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first connection path, the firstconnection path does not include a second connection path, the secondconnection path includes a connection path through the transistor, adrain (or a second terminal or the like) of the transistor iselectrically connected to Y through at least Z2 on a third connectionpath, and the third connection path does not include the secondconnection path”, and “a source (or a first terminal or the like) of atransistor is electrically connected to X through at least Z1 on a firstelectrical path, the first electrical path does not include a secondelectrical path, the second electrical path is an electrical path fromthe source (or the first terminal or the like) of the transistor to adrain (or a second terminal or the like) of the transistor, the drain(or the second terminal or the like) of the transistor is electricallyconnected to Y through at least Z2 on a third electrical path, the thirdelectrical path does not include a fourth electrical path, and thefourth electrical path is an electrical path from the drain (or thesecond terminal or the like) of the transistor to the source (or thefirst terminal or the like) of the transistor”. When the connection pathin a circuit structure is defined by an expression similar to the aboveexamples, a source (or a first terminal or the like) and a drain (or asecond terminal or the like) of a transistor can be distinguished fromeach other to specify the technical scope.

Note that one embodiment of the present invention is not limited tothese expressions which are just examples. Here, each of X, Y, Z1, andZ2 denotes an object (e.g., a device, an element, a circuit, a wiring,an electrode, a terminal, a conductive film, a layer, or the like).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention is described with reference to drawings.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor101 of one embodiment of the present invention. FIG. 1A is the top view.FIG. 1B illustrates a cross section taken along dashed-dotted line A1-A2in FIG. 1A. FIG. 1C illustrates a cross section taken alongdashed-dotted line A3-A4 in FIG. 1A. In FIGS. 1A to 1C, some componentsare enlarged, reduced in size, or omitted for easy understanding. Insome cases, the direction of the dashed-dotted line A1-A2 is referred toas a channel length direction, and the direction of the dashed-dottedline A3-A4 is referred to as a channel width direction.

The transistor 101 includes a base insulating film 120 formed over asubstrate 110, intermediate layers 125 formed over the base insulatingfilm, a source electrode layer 140 and a drain electrode layer 150formed over the intermediate layers, an oxide semiconductor layer 130electrically connected to the source electrode layer and the drainelectrode layer, a gate insulating film 160 formed over the oxidesemiconductor layer, and a gate electrode layer 170 formed over the gateinsulating film. In addition, an insulating layer 180 may be providedover the gate insulating film 160 and the gate electrode layer 170.Furthermore, an insulating layer 185 formed using an oxide may be formedover the insulating layer 180. The insulating layers may be provided asneeded and another insulating layer may be further provided thereover.

Note that functions of a “source” and a “drain” of a transistor aresometimes replaced with each other when a transistor of oppositepolarity is used or when the direction of current flow is changed incircuit operation, for example. Therefore, the terms “source” and“drain” can be replaced with each other in this specification.

Note that at least part (or the whole) of the source electrode layer 140(and/or the drain electrode layer 150) is provided on at least part (orthe whole) of a surface, a side surface, a top surface, and/or a bottomsurface of a semiconductor layer such as the oxide semiconductor layer130.

Alternatively, at least part (or the whole) of the source electrodelayer 140 (and/or the drain electrode layer 150) is in contact with atleast part (or the whole) of a surface, a side surface, a top surface,and/or a bottom surface of a semiconductor layer such as the oxidesemiconductor layer 130. Alternatively, at least part (or the whole) ofthe source electrode layer 140 (and/or the drain electrode layer 150) isin contact with at least part (or the whole) of a semiconductor layersuch as the oxide semiconductor layer 130.

Alternatively, at least part (or the whole) of the source electrodelayer 140 (and/or the drain electrode layer 150) is electricallyconnected to at least part (or the whole) of a surface, a side surface,a top surface, and/or a bottom surface of a semiconductor layer such asthe oxide semiconductor layer 130. Alternatively, at least part (or thewhole) of the source electrode layer 140 (and/or the drain electrodelayer 150) is electrically connected to at least part (or the whole) ofa semiconductor layer such as the oxide semiconductor layer 130.

Alternatively, at least part (or the whole) of the source electrodelayer 140 (and/or the drain electrode layer 150) is provided near atleast part (or the whole) of a surface, a side surface, a top surface,and/or a bottom surface of a semiconductor layer such as the oxidesemiconductor layer 130. Alternatively, at least part (or the whole) ofthe source electrode layer 140 (and/or the drain electrode layer 150) isprovided near at least part (or the whole) of a semiconductor layer suchas the oxide semiconductor layer 130.

Alternatively, at least part (or the whole) of the source electrodelayer 140 (and/or the drain electrode layer 150) is provided next to atleast part (or the whole) of a surface, a side surface, a top surface,and/or a bottom surface of a semiconductor layer such as the oxidesemiconductor layer 130. Alternatively, at least part (or the whole) ofthe source electrode layer 140 (and/or the drain electrode layer 150) isprovided next to at least part (or the whole) of a semiconductor layersuch as the oxide semiconductor layer 130.

Alternatively, at least part (or the whole) of the source electrodelayer 140 (and/or the drain electrode layer 150) is provided obliquelyabove at least part (or the whole) of a surface, a side surface, a topsurface, and/or a bottom surface of a semiconductor layer such as theoxide semiconductor layer 130. Alternatively, at least part (or thewhole) of the source electrode layer 140 (and/or the drain electrodelayer 150) is provided obliquely above at least part (or the whole) of asemiconductor layer such as the oxide semiconductor layer 130.

Alternatively, at least part (or the whole) of the source electrodelayer 140 (and/or the drain electrode layer 150) is provided above atleast part (or the whole) of a surface, a side surface, a top surface,and/or a bottom surface of a semiconductor layer such as the oxidesemiconductor layer 130. Alternatively, at least part (or the whole) ofthe source electrode layer 140 (and/or the drain electrode layer 150) isprovided above at least part (or the whole) of a semiconductor layersuch as the oxide semiconductor layer 130.

A surface of the oxide semiconductor layer 130 that is opposite to asurface in contact with the gate insulating film 160, for example, ispartly in contact with the base insulating film 120. Furthermore,surfaces of the source electrode layer 140 and the drain electrode layer150 that are opposite to surfaces in contact with the oxidesemiconductor layer 130, for example, are in contact with theintermediate layers 125.

An oxygen vacancy in the oxide semiconductor layer 130 contributes tothe formation of a donor; thus, a material including as few oxygenvacancies as possible is preferably used for the oxide semiconductorlayer 130. However, even when the oxide semiconductor layer 130 includesfew oxygen vacancies initially, oxygen vacancies will increase in numberfrom various causes in a manufacturing process of the transistor. Anincrease in the number of oxygen vacancies in an oxide semiconductorlayer leads to poor electrical characteristics in some cases.

In order to compensate an oxygen vacancy in the oxide semiconductorlayer 130, it is preferable that an oxide insulating layer used as thebase insulating film 120 have an oxygen content higher than that in thestoichiometric composition so that oxygen is easily supplied to theoxide semiconductor layer 130 from the base insulating film 120.

However, metal layers used as a source electrode layer and a drainelectrode layer are mostly materials which are easily oxidized. In aconventional structure in which the base insulating film is in contactwith the source electrode layer and the drain electrode layer, there isa problem in that the base insulating film is easily deprived of oxygenby the metal layers and oxygen cannot be sufficiently supplied to theoxide semiconductor layer.

This problem is shown by an experiment using thermal desorptionspectrometry (TDS). A method and results of the experiment are describedbelow.

FIG. 2A is a top view of a top-gate transistor. The transistor includesa base insulating film 520 (not illustrated) over a substrate 510 (notillustrated); an oxide semiconductor layer 530 over the base insulatingfilm; a source electrode layer 540 and a drain electrode layer 550 overthe base insulating film and the oxide semiconductor layer; a gateinsulating film 560 (not illustrated) over the oxide semiconductorlayer, the source electrode layer, and the drain electrode layer; a gateelectrode layer 570 over the gate insulating film; and an insulatingfilm 580 (not illustrated) over the gate insulating film and the gateelectrode layer.

FIG. 2B illustrates specific structures of stacks corresponding toregions 601 to 605 represented by circles in FIG. 2A. For example, inthe region 601, as the base insulating film 520 over the substrate 510,the oxide semiconductor layer (OS) 530, the source electrode layer 540,the gate insulating film 560, the gate electrode layer 570, and theinsulating film 580, a silicon oxynitride film that is an oxideinsulating layer containing excess oxygen, an IGZO film, a tungstenfilm, a silicon oxynitride film, a stacked film of tantalum nitride andtungsten, and a stacked film of an aluminum oxide film and a siliconoxynitride film are formed, respectively, in this order.

In the experiment, samples with stacked-layer structures correspondingto the regions 601 to 605 in FIG. 2B are fabricated. A silicon waferwhose surface is provided with a thermal oxidation film is used as asubstrate. The TDS analysis is performed after etching off upper layersof the samples while leaving the film corresponding to the baseinsulating film 520.

FIG. 3 shows the results of the above experiment. In FIG. 3, the samplescorresponding to the regions 601 to 605 are compared with each other inion intensity that shows the amount of released NO (M/z=30) and ionintensity that shows the amount of released O₂ (M/z=32). As is apparentfrom FIG. 3, in the samples in which the film corresponding to the baseinsulating film 520 and the film corresponding to the source electrodelayer 540 are in contact with each other, the amount of NO and O₂released from the film corresponding to the base insulating film 520 isextremely small. That is, it is suggested that the base insulating film520 is deprived of oxygen by the source electrode layer 540 and thedrain electrode layer 550.

Thus, in one embodiment of the present invention, as illustrated inFIGS. 1A and 1B, in a structure in which a first surface of the oxidesemiconductor layer 130 is in contact with the gate insulating film 160and a second surface that is opposite to the first surface is in contactwith the source electrode layer 140 and the drain electrode layer 150,the intermediate layer 125 is provided between the base insulating film120 and each of the source electrode layer 140 and the drain electrodelayer 150. Owing to the intermediate layer, the base insulating film 120and each of the source electrode layer 140 and the drain electrode layer150 are not in contact with each other; thus, the base insulating film120 is not deprived of oxygen by the source electrode layer 140 and thedrain electrode layer 150, and oxygen is efficiently supplied to theoxide semiconductor layer 130.

For the intermediate layer 125, a material that has a high effect ofblocking diffusion of oxygen from the base insulating film 120 into thesource electrode layer 140 and the drain electrode layer 150 or amaterial that is not easily bonded to oxygen can be used. For example, asingle layer or stacked layers of one or more materials selected fromaluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride,yttrium oxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride,yttria-stabilized zirconia (YSZ), titanium nitride, tantalum nitride,gold, platinum, palladium, and ruthenium can be used. In the case wherea conductive material is used for the intermediate layer 125, theintermediate layer 125 also serves as part of the source electrode layer140 or the drain electrode layer 150.

The patterning for forming the source electrode layer 140 and the drainelectrode layer 150 and the patterning for forming the intermediatelayers 125 apart from each other can be performed in succession. Thatis, the intermediate layers 125 can be formed using a resist mask usedfor the patterning for forming the source electrode layer 140 and thedrain electrode layer 150 and the source electrode layer 140 and thedrain electrode layer 150 themselves as a mask; thus, the number oflithography steps is not increased.

In this case, top surface areas of the intermediate layers 125 aresubstantially the same as top surface areas of the source electrodelayer 140 and the drain electrode layer 150. Furthermore, in the casewhere end portions of the intermediate layers 125, the source electrodelayer 140, and the drain electrode layer 150 have tapered shapes asillustrated in FIGS. 1A and 1B, the top surface areas of theintermediate layers 125 are larger than the top surface areas of thesource electrode layer 140 and the drain electrode layer 150. Note thatin the case where the intermediate layers 125, the source electrodelayer 140, and the drain electrode layer 150 are partly processed orvariation occurs in the etching steps after the formation process ofthese layers, the top surface areas of the intermediate layers 125 donot satisfy the above relation in some cases.

The transistor of one embodiment of the present invention may have astructure illustrated in FIGS. 4A to 4C. FIG. 4A is a top view and FIG.4B illustrates a cross section taken along dashed-dotted line B1-B2 inFIG. 4A. FIG. 4C illustrates a cross section taken along dashed-dottedline B3-B4 in FIG. 4A. In FIGS. 4A to 4C, some components are enlarged,reduced in size, or omitted for easy understanding.

A transistor 102 shown in FIGS. 4A to 4C differs from the transistor 101in that a first oxide semiconductor layer 131, a second oxidesemiconductor layer 132, and a third oxide semiconductor layer 133 areformed, as the oxide semiconductor layer 130, in this order from thebase insulating film 120 side.

The transistor of one embodiment of the present invention may have astructure illustrated in FIGS. 5A to 5C. FIG. 5A is a top view and FIG.5B illustrates a cross section taken along dashed-dotted line C1-C2 inFIG. 5A. FIG. 5C illustrates a cross section taken along dashed-dottedline C3-C4 in FIG. 5A. In FIGS. 5A to 5C, some components are enlarged,reduced in size, or omitted for easy understanding.

A transistor 103 shown in FIGS. 5A to 5C differs from the transistor 101and the transistor 102 in that the oxide semiconductor layer 130includes a stack in which the first oxide semiconductor layer 131 andthe second oxide semiconductor layer 132 are formed in this order fromthe base insulating film 120 side and the third oxide semiconductorlayer 133 covering part of the stack.

In the above transistors 102 and 103, for example, oxide semiconductorfilms with different compositions can be used as the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133.

Next, the components of the transistor of one embodiment of the presentinvention will be described in detail.

The substrate 110 is not limited to a simple supporting substrate, andmay be a substrate where another device such as a transistor is formed.In that case, one of the gate electrode layer 170, the source electrodelayer 140, and the drain electrode layer 150 of the transistor may beelectrically connected to the above device.

The base insulating film 120 can have a function of supplying oxygen tothe oxide semiconductor layer 130 as well as a function of preventingdiffusion of impurities from the substrate 110. For this reason, thebase insulating film 120 is preferably an insulating film containingoxygen and further preferably, the base insulating film 120 is aninsulating film having an oxygen content higher than that in thestoichiometric composition. For example, the base insulating film 120 isa film of which the amount of released oxygen converted into oxygenatoms is 1.0×10¹⁹ atoms/cm³ or more in TDS analysis. Note that thetemperature of the film surface in the TDS analysis is preferably higherthan or equal to 100° C. and lower than or equal to 700° C., or higherthan or equal to 100° C. and lower than or equal to 500° C. In the casewhere the substrate 110 is provided with another device as describedabove, the base insulating film 120 also has a function as an interlayerinsulating film. In that case, the base insulating film 120 ispreferably subjected to planarization treatment such as chemicalmechanical polishing (CMP) treatment so as to have a flat surface.

As the intermediate layer 125, for example, a single layer or stackedlayers of one or more materials selected from aluminum oxide, aluminumoxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttriumoxynitride, hafnium oxide, hafnium oxynitride, yttria-stabilizedzirconia (YSZ), titanium nitride, tantalum nitride, gold, platinum,palladium, and ruthenium can be used, as described above.

For the source electrode layer 140 and the drain electrode layer 150, aconductive material that is easily bonded to oxygen is preferably used.For example, Al, Cr, Cu, Ta, Ti, Mo, or W can be used. Among thematerials, in particular, it is preferable to use Ti that is easilybonded to oxygen or to use W with a high melting point, which allowssubsequent process temperatures to be relatively high. Note that theconductive material that is easily bonded to oxygen includes, in itscategory, a material to which oxygen is easily diffused.

When the conductive material that is easily bonded to oxygen is incontact with an oxide semiconductor layer, a phenomenon occurs in whichoxygen in the oxide semiconductor layer is diffused to the conductivematerial that is easily bonded to oxygen. Oxygen vacancies are generatedin the vicinity of a region which is in the oxide semiconductor layerand is in contact with the source electrode layer or the drain electrodelayer. Hydrogen slightly contained in the film enters into the oxygenvacancies, whereby the region is markedly changed to an n-type region.Accordingly, the n-type region can serve as a source or a drain of thetransistor.

In the transistor 101 in FIGS. 1A to 1C, in the channel formationregion, the oxide semiconductor layer 130 is a single layer. In thetransistor 102 in FIGS. 4A to 4C, the oxide semiconductor layer 130 hasa three-layer structure in which the first oxide semiconductor layer131, the second oxide semiconductor layer 132, and the third oxidesemiconductor layer 133 are stacked in this order from the substrate 110side. In the transistor 103 in FIGS. 5A to 5C, the oxide semiconductorlayer 130 has a three-layer structure as in the transistor 102; however,the transistor 103 is different from the transistor 102 in that thesecond oxide semiconductor layer 132 is surrounded by the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133.

Here, for the second oxide semiconductor layer 132, for example, anoxide semiconductor whose electron affinity (an energy differencebetween a vacuum level and the conduction band minimum) is higher thanthose of the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 is used. The electron affinity can be obtainedby subtracting an energy difference between the conduction band minimumand the valence band maximum (what is called an energy gap) from anenergy difference between the vacuum level and the valence band maximum(what is called an ionization potential).

In this embodiment, detailed description is given mainly on the casewhere the oxide semiconductor layer 130 has a three-layer structure;however, there is no limitation on the number of stacked layers. In thecase where the oxide semiconductor layer 130 is a single layer as inFIGS. 1A to 1C, a layer corresponding to the second oxide semiconductorlayer 132 described in this embodiment is used. In the case where theoxide semiconductor layer 130 has a two-layer structure, for example, astructure of the oxide semiconductor layer 130 in FIGS. 4A to 4C orFIGS. 5A to 5C without the third oxide semiconductor layer 133 isemployed. In such a case, the second oxide semiconductor layer 132 andthe first oxide semiconductor layer 131 can be replaced with each other.In the case where the oxide semiconductor layer 130 has a stacked-layerstructure of four or more layers, for example, a structure in whichanother oxide semiconductor layer is stacked over the three-layer stackdescribed in this embodiment or a structure in which another oxidesemiconductor layer is inserted in any one of the interfaces in thethree-layer stack can be employed.

The first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 each contain one or more kinds of metal elementscontained in the second oxide semiconductor layer 132. For example, thefirst oxide semiconductor layer 131 and the third oxide semiconductorlayer 133 are preferably formed using an oxide semiconductor whoseconduction band minimum is closer to a vacuum level than that of thesecond oxide semiconductor layer 132 by 0.05 eV or more, 0.07 eV ormore, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less,0.5 eV or less, or 0.4 eV or less.

In such a structure, when an electric field is applied to the gateelectrode layer 170, a channel is formed in the second oxidesemiconductor layer 132 whose conduction band minimum is the lowest inthe oxide semiconductor layer 130.

Further, since the first oxide semiconductor layer 131 contains one ormore kinds of metal elements contained in the second oxide semiconductorlayer 132, an interface state is unlikely to be formed at the interfacebetween the second oxide semiconductor layer 132 and the first oxidesemiconductor layer 131, compared with the interface between the secondoxide semiconductor layer 132 and the base insulating film 120 on theassumption that the second oxide semiconductor layer 132 is in contactwith the base insulating film 120. The interface state sometimes forms achannel; therefore, the threshold voltage of the transistor is changedin some cases. Thus, with the first oxide semiconductor layer 131,fluctuations in electrical characteristics of the transistor, such as athreshold voltage, can be reduced. Further, the reliability of thetransistor can be improved.

Furthermore, since the third oxide semiconductor layer 133 contains oneor more kinds of metal elements contained in the second oxidesemiconductor layer 132, scattering of carriers is unlikely to occur atthe interface between the second oxide semiconductor layer 132 and thethird oxide semiconductor layer 133, compared with the interface betweenthe second oxide semiconductor layer 132 and the gate insulating film160 on the assumption that the second oxide semiconductor layer 132 isin contact with the gate insulating film 160. Thus, with the third oxidesemiconductor layer 133, the field-effect mobility of the transistor canbe increased.

For the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133, for example, a material containing Al, Ti, Ga,Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that usedfor the second oxide semiconductor layer 132 can be used. Specifically,an atomic ratio of any of the above metal elements in the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133 is1.5 times or more, preferably 2 times or more, further preferably 3times or more as much as that in the second oxide semiconductor layer132. Any of the above metal elements is strongly bonded to oxygen andthus has a function of suppressing generation of an oxygen vacancy inthe first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133. That is, an oxygen vacancy is less likely to begenerated in the first oxide semiconductor layer 131 and the third oxidesemiconductor layer 133 than in the second oxide semiconductor layer132.

Note that when each of the first oxide semiconductor layer 131, thesecond oxide semiconductor layer 132, and the third oxide semiconductorlayer 133 is an In-M-Zn oxide containing at least indium, zinc, and M (Mis a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and thefirst oxide semiconductor layer 131 has an atomic ratio of In to M andZn which is x₁:y₁:z₁, the second oxide semiconductor layer 132 has anatomic ratio of In to M and Zn which is x₂:y₂:z₂, and the third oxidesemiconductor layer 133 has an atomic ratio of In to M and Zn which isx₃:y₃:z₃, each of y₁/x₁ and y₃/x₃ is preferably larger than y₂/x₂. Eachof y₁/x₁ and y₃/x₃ is 1.5 times or more, preferably 2 times or more,further preferably 3 times or more as large as y₂/x₂. At this time, wheny₂ is greater than or equal to x₂ in the second oxide semiconductorlayer 132, the transistor can have stable electrical characteristics.However, when y₂ is 3 times or more as large as x₂, the field-effectmobility of the transistor is reduced; accordingly, y₂ is preferablysmaller than 3 times x₂.

In the case where Zn and O are not taken into consideration, theproportion of In and the proportion of M in each of the first oxidesemiconductor layer 131 and the third oxide semiconductor layer 133 arepreferably less than 50 atomic % and greater than or equal to 50 atomic%, respectively, further preferably less than 25 atomic % and greaterthan or equal to 75 atomic %, respectively. Further, in the case whereZn and O are not taken into consideration, the proportion of In and theproportion of M in the second oxide semiconductor layer 132 arepreferably greater than or equal to 25 atomic % and less than 75 atomic%, respectively, further preferably greater than or equal to 34 atomic %and less than 66 atomic %, respectively.

The thicknesses of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133 are each greater than or equal to 1 nm andless than or equal to 120 nm, preferably greater than or equal to 3 nmand less than or equal to 50 nm. The thickness of the second oxidesemiconductor layer 132 is greater than or equal to 1 nm and less thanor equal to 200 nm, preferably greater than or equal to 3 nm and lessthan or equal to 100 nm, further preferably greater than or equal to 3nm and less than or equal to 50 nm.

Note that in order that a transistor in which an oxide semiconductorlayer serves as a channel have stable electrical characteristics, it iseffective to reduce the concentration of impurities in the oxidesemiconductor layer to make the oxide semiconductor layer intrinsic orsubstantially intrinsic. The term “substantially intrinsic” refers tothe state where an oxide semiconductor layer has a carrier density whichis lower than 1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, furtherpreferably lower than 1×10¹³/cm³, particularly preferably lower than8×10¹¹/cm³, still further preferably lower than 1×10¹¹/cm³, yet furtherpreferably lower than 1×10¹⁰/cm³, and is 1×10⁻⁹/cm³ or higher.

In the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon,and a metal element other than main components of the oxidesemiconductor layer are impurities. For example, hydrogen and nitrogenform donor levels to increase the carrier density. In addition, siliconin the oxide semiconductor layer forms an impurity level. The impuritylevel serves as a trap and might cause deterioration of electricalcharacteristics of the transistor. Accordingly, in the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133 and at interfaces between theselayers, the impurity concentration is preferably reduced.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, in secondary ion mass spectrometry (SIMS), forexample, the concentration of silicon at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer islower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,further preferably lower than 1×10¹⁸ atoms/cm³. Further, theconcentration of hydrogen at a certain depth of the oxide semiconductorlayer or in a region of the oxide semiconductor layer is lower than orequal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³,still further preferably lower than or equal to 5×10¹⁸ atoms/cm³.Further, the concentration of nitrogen at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer islower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³,still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In the case where the oxide semiconductor layer includes crystals, highconcentration of silicon or carbon might reduce the crystallinity of theoxide semiconductor layer. In order not to lower the crystallinity ofthe oxide semiconductor layer, for example, the concentration of siliconat a certain depth of the oxide semiconductor layer or in a region ofthe oxide semiconductor layer may be lower than 1×10¹⁹ atoms/cm³,preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than1×10¹⁸ atoms/cm³. Further, the concentration of carbon at a certaindepth of the oxide semiconductor layer or in a region of the oxidesemiconductor layer may be lower than 1×10¹⁹ atoms/cm³, preferably lowerthan 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³,for example.

A transistor in which a highly purified oxide semiconductor layer isused for a channel formation region as described above has an extremelylow off-state current. For example, in the case where the voltagebetween the source and the drain is set to approximately 0.1 V, 5 V, or10 V, the off-state current standardized on the channel width of thetransistor can be as low as several yoctoamperes per micrometer toseveral zeptoamperes per micrometer.

Note that as the gate insulating film of the transistor, an insulatingfilm containing silicon is used in many cases; thus, it is preferablethat, as in the transistor of one embodiment of the present invention, aregion of the oxide semiconductor layer, which serves as a channel, notbe in contact with the gate insulating film for the above-describedreason. In the case where a channel is formed at the interface betweenthe gate insulating film and the oxide semiconductor layer, scatteringof carriers occurs at the interface, whereby the field-effect mobilityof the transistor is reduced in some cases. Also from the view of theabove, it is preferable that the region of the oxide semiconductorlayer, which serves as a channel, be separated from the gate insulatingfilm.

Accordingly, with the oxide semiconductor layer 130 having astacked-layer structure including the first oxide semiconductor layer131, the second oxide semiconductor layer 132, and the third oxidesemiconductor layer 133, a channel can be formed in the second oxidesemiconductor layer 132; thus, the transistor can have a highfield-effect mobility and stable electrical characteristics.

In a band structure, the conduction band minimums of the first oxidesemiconductor layer 131, the second oxide semiconductor layer 132, andthe third oxide semiconductor layer 133 are continuously changed. Thiscan be understood also from the fact that the compositions of the firstoxide semiconductor layer 131, the second oxide semiconductor layer 132,and the third oxide semiconductor layer 133 are close to one another andoxygen is easily diffused among the first oxide semiconductor layer 131,the second oxide semiconductor layer 132, and the third oxidesemiconductor layer 133. Thus, the first oxide semiconductor layer 131,the second oxide semiconductor layer 132, and the third oxidesemiconductor layer 133 have a continuous physical property althoughthey have different compositions and form a stack. In the drawings inthis specification, interfaces between the oxide semiconductor layers ofthe stack are indicated by dotted lines.

The oxide semiconductor layer 130 in which layers containing the samemain components are stacked is formed to have not only a simplestacked-layer structure of the layers but also a continuous energy band(here, in particular, a well structure having a U shape in which theconduction band minimums are continuously changed (U-shaped well)). Inother words, the stacked-layer structure is formed such that thereexists no impurity that forms a defect level such as a trap center or arecombination center at each interface. If impurities exist between thestacked oxide semiconductor layers, the continuity of the energy band islost and carriers disappear by a trap or recombination at the interface.

For example, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:3:2, 1:3:3, 1:3:4, 1:3:6, 1:6:4, or 1:9:6 can be used for the firstoxide semiconductor layer 131 and the third oxide semiconductor layer133 and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is1:1:1, 5:5:6, or 3:1:2 can be used for the second oxide semiconductorlayer 132.

The second oxide semiconductor layer 132 of the oxide semiconductorlayer 130 serves as a well, so that a channel is formed in the secondoxide semiconductor layer 132 in a transistor including the oxidesemiconductor layer 130. Note that since the conduction band minimumsare continuously changed, the oxide semiconductor layer 130 can also bereferred to as a U-shaped well. Further, a channel formed to have such astructure can also be referred to as a buried channel.

Note that trap levels due to impurities or defects might be formed inthe vicinity of the interface between an insulating film such as asilicon oxide film and each of the first oxide semiconductor layer 131and the third oxide semiconductor layer 133. The second oxidesemiconductor layer 132 can be distanced away from the trap levels owingto existence of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133.

However, when the energy differences between the conduction band minimumof the second oxide semiconductor layer 132 and the conduction bandminimum of each of the first oxide semiconductor layer 131 and the thirdoxide semiconductor layer 133 are small, an electron in the second oxidesemiconductor layer 132 might reach the trap level by passing over theenergy differences. The electron is trapped by the trap level to benegative charge, whereby the threshold voltage of the transistor shiftsin the positive direction.

Thus, to reduce fluctuations in the threshold voltage of the transistor,energy differences of at least certain values between the conductionband minimum of the second oxide semiconductor layer 132 and theconduction band minimum of each of the first oxide semiconductor layer131 and the third oxide semiconductor layer 133 are necessary. Each ofthe energy differences is preferably greater than or equal to 0.1 eV,further preferably greater than or equal to 0.15 eV.

The first oxide semiconductor layer 131, the second oxide semiconductorlayer 132, and the third oxide semiconductor layer 133 preferablyinclude crystal parts. In particular, when crystals with c-axisalignment are used, the transistor can have stable electricalcharacteristics.

The gate insulating film 160 can be formed using, for example, aninsulating film containing one or more of aluminum oxide, magnesiumoxide, silicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. Thegate insulating film 160 may be a stack of any of the above materials.The gate insulating film 160 may contain lanthanum (La), nitrogen, orzirconium (Zr) as an impurity.

An example of a stacked-layer structure of the gate insulating film 160will be described. The gate insulating film 160 includes, for example,oxygen, nitrogen, silicon, or hafnium. Specifically, the gate insulatingfilm 160 preferably includes hafnium oxide and silicon oxide or siliconoxynitride.

Hafnium oxide has higher dielectric constant than silicon oxide andsilicon oxynitride. Therefore, by using hafnium oxide, a physicalthickness can be made larger than an equivalent oxide thickness; thus,even in the case where the equivalent oxide thickness is less than orequal to 10 nm or less than or equal to 5 nm, leakage current due totunnel current can be low. That is, it is possible to provide atransistor with a low off-state current. Moreover, hafnium oxide with acrystalline structure has higher dielectric constant than hafnium oxidewith an amorphous structure. Therefore, it is preferable to use hafniumoxide with a crystalline structure in order to provide a transistor witha low off-state current. Examples of the crystalline structure include amonoclinic crystal structure and a cubic crystal structure. Note thatone embodiment of the present invention is not limited to the aboveexamples.

In some cases, an interface state due to a defect exists in hafniumoxide having a crystalline structure in a formation surface where thehafnium oxide having the crystalline structure is formed. The interfacestate serves as a trap center in some cases. Therefore, when hafniumoxide is provided near a channel region of a transistor, the electricalcharacteristics of the transistor might deteriorate because of theinterface state. In order to reduce the adverse effect of the interfacestate, in some cases, it is preferable to separate the channel region ofthe transistor and the hafnium oxide from each other by providinganother film therebetween. The film has a buffer function. The filmhaving a buffer function may be included in the gate insulating film 160or included in the oxide semiconductor layer 130. That is, the filmhaving a buffer function can be formed using silicon oxide, siliconoxynitride, an oxide semiconductor, or the like. Note that the filmhaving a buffer function is formed using, for example, a semiconductoror an insulator having a larger energy gap than a semiconductor to bethe channel region. Alternatively, the film having a buffer function isformed using, for example, a semiconductor or an insulator having lowerelectron affinity than a semiconductor to be the channel region. Furtheralternatively, the film having a buffer function is formed using, forexample, a semiconductor or an insulator having higher ionization energythan a semiconductor to be the channel region.

In some cases, the threshold voltage of a transistor can be controlledby trapping an electric charge in an interface state (trap center) inhafnium oxide having the above-described crystalline structure in theformation surface where the hafnium oxide having the above-describedcrystalline structure is formed. In order to make the electric chargeexist stably, for example, an insulator having a larger energy gap thanhafnium oxide may be provided between the channel region and the hafniumoxide. Alternatively, a semiconductor or an insulator having lowerelectron affinity than hafnium oxide may be provided. The film having abuffer function may be formed using a semiconductor or an insulatorhaving higher ionization energy than hafnium oxide. With the use of sucha semiconductor or an insulator, an electric charge trapped in theinterface state is less likely to be released; accordingly, the electriccharge can be held for a long period of time.

Examples of such an insulator include silicon oxide and siliconoxynitride. In order to make the interface state in the gate insulatingfilm 160 trap an electric charge, an electron may be transferred fromthe oxide semiconductor layer 130 toward the gate electrode layer 170.As a specific example, the potential of the gate electrode layer 170 iskept higher than the potential of the source electrode layer 140 or thedrain electrode layer 150 under high temperature conditions (e.g., atemperature higher than or equal to 125° C. and lower than or equal to450° C., typically higher than or equal to 150° C. and lower than orequal to 300° C.) for one second or longer, typically for one minute orlonger.

The threshold voltage of a transistor in which a predetermined amount ofelectrons are trapped in interface states in the gate insulating film160 or the like shifts in the positive direction. The amount ofelectrons to be trapped (the amount of change in threshold voltage) canbe controlled by adjusting a voltage of the gate electrode layer 170 ortime in which the voltage is applied. Note that a location in which anelectric charge is trapped is not necessarily limited to the inside ofthe gate insulating film 160 as long as an electric charge can betrapped therein. A stacked film having a similar structure may be usedas another insulating layer.

For the gate electrode layer 170, a conductive film formed using Al, Ti,Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used. Thegate electrode layer may be a stack of any of the above materials.Alternatively, a conductive film containing nitrogen, such as a nitrideof the above material, may be used for the gate electrode layer.

An aluminum oxide film is preferably included in the insulating layer180 over the gate insulating film 160 and the gate electrode layer 170.The aluminum oxide film has a high blocking effect of preventingpenetration of both oxygen and impurities such as hydrogen and moisture.Accordingly, during and after the manufacturing process of thetransistor, the aluminum oxide film can suitably function as aprotective film that has effects of preventing entry of impurities suchas hydrogen and moisture, which cause variations in the electricalcharacteristics of the transistor, into the oxide semiconductor layer130, preventing release of oxygen, which is a main component of theoxide semiconductor layer 130, from the oxide semiconductor layer, andpreventing unnecessary release of oxygen from the base insulating film120. Further, oxygen contained in the aluminum oxide film can bediffused in the oxide semiconductor layer 130.

Further, the insulating layer 185 is preferably formed over theinsulating layer 180. Each of the insulating layers can be formed usingan insulating film containing one or more of magnesium oxide, siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.Each of the insulating layers may be a stack of any of the abovematerials.

Here, like the base insulating film 120, the insulating layer 185preferably contains oxygen more than that in the stoichiometriccomposition. Oxygen released from the insulating layer 185 can bediffused into the channel formation region in the oxide semiconductorlayer 130 through the gate insulating film 160, so that oxygen vacanciesformed in the channel formation region can be filled with the oxygen. Inthis manner, stable electrical characteristics of the transistor can beachieved.

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of electrical characteristics of the transistor. Adecrease in channel width causes a reduction in on-state current.

In the transistor of one embodiment of the present invention shown inFIGS. 5A to 5C, for example, as described above, the third oxidesemiconductor layer 133 is formed so as to cover the second oxidesemiconductor layer 132 where a channel is formed and the channelformation layer and the gate insulating film are not in contact witheach other. Accordingly, scattering of carriers at the interface betweenthe channel formation layer and the gate insulating film can be reducedand the on-state current of the transistor can be increased.

In the transistor of one embodiment of the present invention, the gateelectrode layer 170 is formed to electrically surround the oxidesemiconductor layer 130 in the channel width direction; accordingly, agate electric field is applied to the oxide semiconductor layer 130 inthe side surface direction in addition to the perpendicular direction.In other words, a gate electric field is applied to the oxidesemiconductor layer entirely, so that current flows in the whole of thesecond oxide semiconductor layer 132 serving as a channel, leading to afurther increase in on-state current.

In the transistor of one embodiment of the present invention, the secondoxide semiconductor layer 132 is formed over the first oxidesemiconductor layer 131, so that an interface state is less likely to beformed. In addition, impurities do not enter the second oxidesemiconductor layer 132 from above and below because the second oxidesemiconductor layer 132 is positioned at the middle of the three-layerstructure. Therefore, the transistor can achieve not only the increasein the on-state current of the transistor but also stabilization of thethreshold voltage and a reduction in the S value (subthreshold value).Thus, Icut (current when gate voltage VG is 0 V) can be reduced andpower consumption can be reduced. Further, since the threshold voltageof the transistor becomes stable, long-term reliability of thesemiconductor device can be improved.

The transistor of one embodiment of the present invention may include aconductive film 172 between the oxide semiconductor layer 130 and thesubstrate 110 as illustrated in FIG. 6A. When the conductive film isused as a second gate electrode layer, the on-state current can befurther increased and the threshold voltage can be controlled. In orderto increase the on-state current, for example, the gate electrode layer170 and the conductive film 172 are set to have the same potential, andthe transistor is driven as a dual-gate transistor. In this case, asshown in FIG. 6B, the gate electrode layer 170 and the conductive film172 may be connected to each other through a contact hole. Furthermore,to control the threshold voltage, a fixed potential, which is differentfrom a potential of the gate electrode layer 170, is supplied to theconductive film 172.

Although an example where a channel or the like is formed in the oxidesemiconductor layer 130 or the like is described in this embodiment, oneembodiment of the present invention is not limited thereto. For example,depending on cases or conditions, a channel, the vicinity of thechannel, a source region, a drain region, or the like may be formedusing a material containing silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, an organicsemiconductor, or the like.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 2

In this embodiment, a method for manufacturing the transistor 102described in Embodiment 1 with reference to FIGS. 4A to 4C is mainlydescribed. In addition, a method for manufacturing the transistor 101 inFIGS. 1A to 1C and a method for manufacturing the transistor 103 inFIGS. 5A to 5C are also described.

For the substrate 110, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like can be used. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon, silicon carbide, or the like, acompound semiconductor substrate of silicon germanium or the like, asemiconductor-on-insulator (SOI) substrate, or the like may be used.Still alternatively, any of these substrates provided with asemiconductor element may be used.

The base insulating film 120 can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating film includingaluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride,gallium oxide, germanium oxide, yttrium oxide, zirconium oxide,lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or thelike; a nitride insulating film including silicon nitride, siliconnitride oxide, aluminum nitride, aluminum nitride oxide, or the like; ora film in which any of the above materials are mixed. Alternatively, astack including any of the above materials may be used, and at least anupper layer of the stack which is in contact with the oxidesemiconductor layer 130 is preferably formed using a material containingexcess oxygen that might serve as a supply source of oxygen to the oxidesemiconductor layer 130.

Oxygen may be added to the base insulating film 120 by an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, or the like. Adding oxygen enables the baseinsulating film 120 to supply oxygen much easily to the oxidesemiconductor layer 130.

In the case where a surface of the substrate 110 is made of an insulatorand there is no influence of impurity diffusion to the oxidesemiconductor layer 130 to be formed later, the base insulating film 120is not necessarily provided.

In the case of the structure of the transistor in FIGS. 6A and 6B, theconductive film 172 is formed before the formation of the baseinsulating film 120.

Then, a thin film 325 to be the intermediate layers 125 and a firstconductive film 340 to be the source electrode layer 140 and the drainelectrode layer 150 are formed over the base insulating film 120 by asputtering method, a CVD method, an MBE method, or the like (see FIG.7A).

As the thin film 325, for example, a single layer or stacked layers ofone or more materials selected from aluminum oxide, aluminum oxynitride,gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride,hafnium oxide, hafnium oxynitride, yttria-stabilized zirconia (YSZ),titanium nitride, tantalum nitride, gold, platinum, palladium, andruthenium can be used. For example, a 10-nm-thick aluminum oxide film isformed by a sputtering method.

For the first conductive film 340, Al, Cr, Cu, Ta, Ti, Mo, W, or analloy material containing any of these as its main component can beused. For example, a tungsten film with a thickness of 100 nm is formedby a sputtering method, a CVD method, or the like.

Next, a first resist mask 401 is formed over the first conductive film340. It is preferable that the first resist mask 401 be formed by alithography method using electron beam exposure, liquid immersionexposure, or EUV exposure, for example. In the case of using electronbeam exposure, using a negative photoresist material for forming thefirst resist mask 401 can shorten the time needed for the light exposurestep. Then, the first conductive film 340 and the thin film 325 areselectively etched using the first resist mask 401 as a mask, so that astack of the source electrode layer 140 and the intermediate layer 125and a stack of the drain electrode layer 150 and the intermediate layer125 are formed (see FIG. 7B). Note that the base insulating film 120 maybe partly etched by over-etching of the thin film 325.

Next, a first oxide semiconductor film 331 to be the first oxidesemiconductor layer 131, a second oxide semiconductor film 332 to be thesecond oxide semiconductor layer 132, and a third oxide semiconductorfilm 333 to be the third oxide semiconductor layer 133 are formed overthe base insulating film 120, the stack of the source electrode layer140 and the intermediate layer 125, and the stack of the drain electrodelayer 150 and the intermediate layer 125 by a sputtering method, a CVDmethod, an MBE method, or the like (see FIG. 7C).

Note that in the case where the transistor 101 in FIGS. 1A to 1C isformed, a single oxide semiconductor layer corresponding to the secondoxide semiconductor layer 132 is provided.

In the case where the oxide semiconductor layer 130 has a stacked-layerstructure, oxide semiconductor films are preferably formed successivelywithout exposure to the air with the use of a multi-chamber depositionapparatus (e.g., a sputtering apparatus) including a load lock chamber.It is preferable that each chamber of the sputtering apparatus be ableto be evacuated to a high vacuum (approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa)by an adsorption vacuum evacuation pump such as a cryopump and that thechamber be able to heat a substrate over which a film is to be depositedto 100° C. or higher, preferably 500° C. or higher, so that water andthe like acting as impurities of an oxide semiconductor are removed asmuch as possible. Alternatively, a combination of a turbo molecular pumpand a cold trap is preferably used to prevent back-flow of a gascontaining a carbon component, moisture, or the like from an exhaustsystem into the chamber. Alternatively, a combination of a turbomolecular pump and a cryopump may be used as an exhaust system.

Not only high vacuum evacuation of the chamber but also high purity of asputtering gas is necessary to obtain a highly purified intrinsic oxidesemiconductor. As an oxygen gas or an argon gas used for a sputteringgas, a gas which is highly purified to have a dew point of −40° C. orlower, preferably −80° C. or lower, further preferably −100° C. or loweris used, whereby entry of moisture or the like into the oxidesemiconductor film can be prevented as much as possible.

For the first oxide semiconductor film 331, the second oxidesemiconductor film 332, and the third oxide semiconductor film 333, anyof the materials described in Embodiment 1 can be used. For example, anIn—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4,1:3:3, or 1:3:2 can be used for the first oxide semiconductor film 331,an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1, or5:5:6 can be used for the second oxide semiconductor film 332, and anIn—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:6, 1:3:4,1:3:3, or 1:3:2 can be used for the third oxide semiconductor film 333.Note that the atomic ratio of each of the first oxide semiconductor film331, the second oxide semiconductor film 332, and the third oxidesemiconductor film 333 may vary within a range of ±20% of the aboveatomic ratio as an error. In the case where a sputtering method is usedfor deposition, the above material can be used as a target.

An oxide semiconductor that can be used for each of the first oxidesemiconductor film 331, the second oxide semiconductor film 332, and thethird oxide semiconductor film 333 preferably contains at least indium(In) or zinc (Zn). Both In and Zn are preferably contained. In order toreduce fluctuations in electrical characteristics of the transistorincluding the oxide semiconductor, the oxide semiconductor preferablycontains a stabilizer in addition to In and Zn.

As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),zirconium (Zr), and the like can be given. As another stabilizer,lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), or lutetium (Lu) can be given.

As the oxide semiconductor, for example, any of the following can beused: indium oxide, gallium oxide, tin oxide, zinc oxide, an In—Znoxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, anIn—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, anIn—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide,an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Znoxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, anIn—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide,an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Znoxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide,an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, and an In—Hf—Al—Zn oxide.

For example, “In—Ga—Zn oxide” means an oxide containing In, Ga, and Znas its main components. The In—Ga—Zn oxide may contain another metalelement in addition to In, Ga, and Zn. Note that in this specification,a film containing the In—Ga—Zn oxide is also referred to as an IGZOfilm.

A material represented by InMO₃(ZnO)_(m) (m>0 is satisfied, and m is notan integer) may be used. Note that M represents one or more metalelements selected from Ga, Y, Zr, La, Ce, and Nd. Alternatively, amaterial represented by In₂SnO₅(ZnO)_(n) (n>0, n is an integer) may beused.

Note that as described in Embodiment 1 in detail, materials are selectedso that the first oxide semiconductor film 331 and the third oxidesemiconductor film 333 each have an electron affinity lower than that ofthe second oxide semiconductor film 332.

Note that the oxide semiconductor films are preferably formed by asputtering method. As a sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used.

In the case of using an In—Ga—Zn oxide, a material whose atomic ratio ofIn to Ga and Zn is any of 1:1:1, 2:2:1, 3:1:2, 5:5:6, 1:3:2, 1:3:3,1:3:4, 1:3:6, 1:4:3, 1:5:4, 1:6:6, 2:1:3, 1:6:4, 1:9:6, 1:1:4, and 1:1:2can be used for each of the first oxide semiconductor film 331, thesecond oxide semiconductor film 332, and the third oxide semiconductorfilm 333 so that the first oxide semiconductor film 331 and the thirdoxide semiconductor film 333 each have an electron affinity lower thanthat of the second oxide semiconductor film 332. In the case where asputtering method is used for deposition, the above material can be usedas a target.

The indium content in the second oxide semiconductor film 332 ispreferably higher than those in the first and third oxide semiconductorfilms 331 and 333. In an oxide semiconductor, the s orbital of heavymetal mainly contributes to carrier transfer, and when the proportion ofIn in the oxide semiconductor is increased, overlap of the s orbitals islikely to be increased. Therefore, an oxide having a composition inwhich the proportion of In is higher than that of Ga has higher mobilitythan an oxide having a composition in which the proportion of In isequal to or lower than that of Ga. Thus, with the use of an oxide havinga high indium content for the second oxide semiconductor layer 132, atransistor having high mobility can be achieved.

First heat treatment may be performed after the third oxidesemiconductor film 333 is formed. The first heat treatment may beperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 300° C. andlower than or equal to 500° C., in an inert gas atmosphere, anatmosphere containing an oxidizing gas at 10 ppm or more, or a reducedpressure state. Alternatively, the first heat treatment may be performedin such a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, in order tocompensate released oxygen. The first heat treatment can increase thecrystallinity of the first to third oxide semiconductor films 331 to 333and remove impurities such as water and hydrogen from the baseinsulating film 120 and the first to third oxide semiconductor films 331to 333. Note that the first heat treatment may be performed afteretching for formation of the first to third oxide semiconductor layers131 to 133, which is described later.

Next, a second resist mask 402 is formed, and with the resist mask, thethird oxide semiconductor film 333, the second oxide semiconductor film332, and the first oxide semiconductor film 331 are selectively etched,whereby the third oxide semiconductor layer 133, the second oxidesemiconductor layer 132, and the first oxide semiconductor layer 131 areformed (see FIG. 8A). It is also possible to use a metal layer as a hardmask to form the third oxide semiconductor layer 133, the second oxidesemiconductor layer 132, and the first oxide semiconductor layer 131.The metal layer is obtained by forming a metal film over the third oxidesemiconductor film 333 and selectively etching the metal film with theuse of the second resist mask 402.

Next, the gate insulating film 160 is formed over the third oxidesemiconductor layer 133. The gate insulating film 160 can be formedusing aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, or the like. The gateinsulating film 160 may be a stack of any of the above materials. Thegate insulating film 160 can be formed by a sputtering method, a CVDmethod, an MBE method, or the like.

Then, a second conductive film 370 (not illustrated) to be the gateelectrode layer 170 is formed over the gate insulating film 160. For thesecond conductive film 370, Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag,Ta, W, or an alloy material containing any of these as its maincomponent can be used, for example. The second conductive film 370 canbe formed by a sputtering method, a CVD method, or the like. A stackincluding a conductive film containing any of the above materials and aconductive film containing nitrogen, or a conductive film containingnitrogen may be used for the second conductive film 370.

After that, a third resist mask 403 is formed over the second conductivefilm 370, and the second conductive film 370 is selectively etched usingthe third resist mask to form the gate electrode layer 170 (see FIG.8B).

Then, the insulating layer 180 and the insulating layer 185 are formedover the source electrode layer 140, the drain electrode layer 150, andthe gate electrode layer 170 (see FIG. 8C). The insulating layer 180 andthe insulating layer 185 can each be formed using a material and amethod which are similar to those of the base insulating film 120. Notethat it is particularly preferable to use aluminum oxide for theinsulating layer 180.

Oxygen may be added to the insulating layer 180 and/or the insulatinglayer 185 by an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, or the like. Adding oxygen enablesthe insulating layer 180 and/or the insulating layer 185 to supplyoxygen much easily to the oxide semiconductor layer 130.

After that, second heat treatment may be performed. The second heattreatment can be performed in a condition similar to that of the firstheat treatment. By the second heat treatment, excess oxygen is easilyreleased from the base insulating film 120, the insulating layer 180,and the insulating layer 185, so that the number of oxygen vacancies inthe oxide semiconductor layer 130 can be reduced.

Through the above steps, the transistor 102 shown in FIGS. 4A to 4C canbe manufactured. In addition, as described above, when a single layer isused as the oxide semiconductor layer 130, the transistor 101 shown inFIGS. 1A to 1C can be manufactured.

Next, a method for manufacturing the transistor 103 illustrated in FIGS.5A to 5C is described. For the steps up to the step of forming thesource electrode layer 140, the drain electrode layer 150, and theintermediate layers 125, the above-described steps illustrated in FIGS.7A and 7B can be referred to. Furthermore, the description of heattreatment steps is omitted.

Then, the first oxide semiconductor film 331 and the second oxidesemiconductor film 332 are formed over the base insulating film 120, thesource electrode layer 140, and the drain electrode layer 150 (see FIG.9A). For this step, the step illustrated in FIG. 7C can be referred to(except for the third oxide semiconductor film 333).

Next, a fourth resist mask 404 is formed, and with the resist mask, thesecond oxide semiconductor film 332 and the first oxide semiconductorfilm 331 are selectively etched, whereby the second oxide semiconductorlayer 132 and the first oxide semiconductor layer 131 are formed (seeFIG. 9B). It is also possible to use a metal layer as a hard mask toform the second oxide semiconductor layer 132 and the first oxidesemiconductor layer 131. The metal layer is obtained by forming a metalfilm over the second oxide semiconductor film 332 and selectivelyetching the metal film with the use of the fourth resist mask 404.

Then, the third oxide semiconductor film 333 is formed to cover thesource electrode layer 140, the drain electrode layer 150, theintermediate layers 125, the second oxide semiconductor layer 132, andthe first oxide semiconductor layer 131 (see FIG. 9C). For this step,the above description of the third oxide semiconductor film 333 can bereferred to.

Next, the gate insulating film 160 is formed over the third oxidesemiconductor film 333. Then, the second conductive film 370 to be thegate electrode layer 170 is formed over the gate insulating film 160(see FIG. 10A). For this step, the description of the gate insulatingfilm 160 and the second conductive film 370 can be referred to.

After that, a fifth resist mask 405 is formed over the second conductivefilm 370, and the second conductive film 370 is selectively etched usingthe resist mask to form the gate electrode layer 170.

Then, the gate insulating film 160 is selectively etched using the gateelectrode layer 170 as a mask.

Subsequently, the third oxide semiconductor film 333, the second oxidesemiconductor layer 132, and the first oxide semiconductor layer 131 areetched using the gate electrode layer 170 or the gate insulating film160 as a mask; thus, the third oxide semiconductor layer 133 is formed(see FIG. 10B). At this time, the second oxide semiconductor layer 132and the first oxide semiconductor layer 131 do not need to be etched.

The second conductive film 370, the gate insulating film 160, the thirdoxide semiconductor film 333, the second oxide semiconductor layer 132,and the first oxide semiconductor layer 131 may be etched individuallyor successively. Either dry etching or wet etching may be used as theetching method, and an appropriate etching method may be selectedindividually.

Alternatively, after the formation of the gate electrode layer 170,another resist mask may be formed over the gate insulating film 160 andthe gate electrode layer 170, and the gate insulating film 160 and thethird oxide semiconductor film 333 may be selectively etched with theresist mask.

Next, the insulating layer 180 and the insulating layer 185 are formedover the source electrode layer 140, the drain electrode layer 150, andthe gate electrode layer 170 (see FIG. 10C). For this step, thedescription of the insulating layer 180 and the insulating layer 185 canbe referred to.

Through the above steps, the transistor 103 shown in FIGS. 5A to 5C canbe manufactured.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films which are described in thisembodiment typically can be formed by a sputtering method or a plasmaCVD method, such films may be formed by another method, e.g., a thermalCVD method. A metal organic chemical vapor deposition (MOCVD) method oran atomic layer deposition (ALD) method may be employed as an example ofa thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first source gas so thatthe source gases are not mixed, and then a second source gas isintroduced. Note that in the case where the first source gas and theinert gas are introduced at a time, the inert gas serves as a carriergas, and the inert gas may also be introduced at the same time as theintroduction of the second source gas. Alternatively, the first sourcegas may be exhausted by vacuum evacuation instead of the introduction ofthe inert gas, and then the second source gas may be introduced. Thefirst source gas is adsorbed on the surface of the substrate to form afirst layer; then the second source gas is introduced to react with thefirst layer; as a result, a second layer is stacked over the firstlayer, so that a thin film is formed. The sequence of the gasintroduction is repeated plural times until a desired thickness isobtained, whereby a thin film with excellent step coverage can beformed. The thickness of the thin film can be adjusted by the number ofrepetition times of the sequence of the gas introduction; therefore, anALD method makes it possible to accurately adjust a thickness and thusis suitable for manufacturing a minute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in theembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O film isformed, trimethylindium, trimethylgallium, and dimethylzinc can be used.Note that the chemical formula of trimethylindium is In(CH₃)₃. Thechemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formulaof dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed with adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source material gas which is obtained by vaporizingliquid containing a solvent and a hafnium precursor compound (a hafniumalkoxide solution, typically tetrakis(dimethylamide)hafnium (TDMAH)) areused. Note that the chemical formula of tetrakis(dimethylamide)hafniumis Hf[N(CH₃)₂]₄. Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed with adeposition apparatus employing ALD, hexachlorodisilane is adsorbed on asurface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Instead of an In(CH₃)₃ gas, an In(C2H₅)₃ gas may be used. Instead of aGa(CH₃)₃ gas, a Ga(C2H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gasmay be used.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 3

In this embodiment, a transistor having a structure which is differentfrom the structures of the transistors 101 to 103 described inEmbodiment 1 and Embodiment 2 and a method for manufacturing thetransistor will be described.

FIGS. 11A to 11C are a top view and cross-sectional views of thetransistor 104 of one embodiment of the present invention. FIG. 11A isthe top view, and FIG. 11B illustrates a cross section taken alongdashed-dotted line D1-D2 in FIG. 11A. FIG. 11C illustrates a crosssection taken along dashed-dotted line D3-D4 in FIG. 11A. FIG. 12illustrates a cross section taken along dashed-dotted line D5-D6 in FIG.11A. In FIGS. 11A to 11C and FIG. 12, some components are enlarged,reduced in size, or omitted for easy understanding. In some cases, thedirection of the dashed-dotted line D1-D2 is referred to as a channellength direction, and the direction of the dashed-dotted line D3-D4 isreferred to as a channel width direction.

In the transistor 104 in FIGS. 11A to 11C and FIG. 12, the cross sectionin the channel length direction (direction of D1-D2) is similar to thatin the transistor 103 in FIGS. 5A to 5C; however, the cross section inthe channel width direction (direction of D3-D4) is different from thatin the transistor 103.

The process for forming a basic structure of the above-describedtransistor 103 includes a first lithography step for forming the sourceelectrode layer 140, the drain electrode layer 150, and the intermediatelayers 125, a second lithography step for forming the stack of the firstoxide semiconductor layer 131 and the second oxide semiconductor layer132, and a third lithography step for forming the gate electrode layer170. Similarly, the manufacturing processes of the transistor 101 andthe transistor 102 each include three lithography steps.

On the other hand, the oxide semiconductor layer 130 of the transistor104 in this embodiment is formed using a mask for forming the gateelectrode layer 170 or the gate electrode layer as a mask in accordancewith a manufacturing method described later. Thus, a basic structure ofthe transistor 104 can be formed by two lithography steps: a firstlithography step for forming the source electrode layer 140, the drainelectrode layer 150, and the intermediate layers 125 and a secondlithography step for forming the gate electrode layer 170 and the oxidesemiconductor layer 130.

That is, the transistor 104 can be manufactured in a simplified process.Oxygen can be efficiently supplied to the oxide semiconductor layer 130owing to formation of the intermediate layers 125 as described inEmbodiment 1, and the transistor can be formed at lower cost.

In the transistor 104 in FIGS. 11A to 11C, the oxide semiconductor layer130 has a three-layer structure of the first oxide semiconductor layer131, the second oxide semiconductor layer 132, and the third oxidesemiconductor 133; however, the oxide semiconductor layer 130 may have asingle-layer structure using a material corresponding to that of thesecond oxide semiconductor layer 132.

The transistor 104 may include a conductive film 172 between the oxidesemiconductor layer 130 and the substrate 110 as in the transistor inFIGS. 6A and 6B.

Furthermore, in the transistor 104, top surface shapes of the gateelectrode layer 170, the gate insulating film 160, and the oxidesemiconductor layer 130 are substantially the same or similar to eachother. Thus, the channel width W of the transistor is defined by not thewidth of the oxide semiconductor layer 130 but the width of the sourceelectrode layer 140 or the width of the drain electrode layer 150 asillustrated in FIG. 12.

Note that the channel length refers to, for example, a distance betweena source (a source region or a source electrode) and a drain (a drainregion or a drain electrode) in a region where a semiconductor (or aportion where a current flows in a semiconductor when a transistor ison) and a gate electrode overlap with each other or a region where achannel is formed in a top view of the transistor. In one transistor,channel lengths in all regions are not necessarily the same. In otherwords, the channel length of one transistor is not limited to one valuein some cases. Therefore, in this specification, the channel length isany one of values, the maximum value, the minimum value, or the averagevalue in a region where a channel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where a current flows in a semiconductor when a transistoris on) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily have the same value. In other words, a channel width ofone transistor is not fixed to one value in some cases. Therefore, inthis specification, a channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of a semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example,estimation of an effective channel width from a design value requires anassumption that the shape of a semiconductor is known. Therefore, in thecase where the shape of a semiconductor is not known accurately, it isdifficult to measure an effective channel width accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where field-effect mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

Next, a method for manufacturing the transistor 104 is described. Forthe steps up to the step of forming the three-layer structure of thefirst oxide semiconductor film 331, the second oxide semiconductor film332, and the third oxide semiconductor film 333, the steps illustratedin FIGS. 7A to 7C in Embodiment 2 can be referred to. In the case wherethe oxide semiconductor layer 130 has a single-layer structure, a singlelayer of a film corresponding to the second oxide semiconductor film 332is formed. Note that description of steps similar to those in themanufacturing method of the transistor 103 is omitted.

Next, the gate insulating film 160 and the second conductive film 370 tobe the gate electrode layer 170 are formed over the third oxidesemiconductor film 333 (see FIG. 13A). For this step, the description ofthe gate insulating film 160 and the second conductive film 370 can bereferred to.

After that, a fifth resist mask 406 is formed over the second conductivefilm 370, and the second conductive film 370 is selectively etched usingthe resist mask to form the gate electrode layer 170.

Then, the gate insulating film 160 is selectively etched using the gateelectrode layer 170 as a mask.

Subsequently, the third oxide semiconductor film 333, the second oxidesemiconductor film 332, and the first oxide semiconductor film 331 areetched using the gate electrode layer 170 or the gate insulating film160 as a mask to form the third oxide semiconductor layer 133, thesecond oxide semiconductor layer 132, and the first oxide semiconductorlayer 131 (see FIG. 13B).

Next, the insulating layer 180 and the insulating layer 185 are formedover the source electrode layer 140, the drain electrode layer 150, andthe gate electrode layer 170 (see FIG. 13C). For this step, thedescription of the insulating layer 180 and the insulating layer 185 canbe referred to.

Through the above steps, the transistor 104 shown in FIGS. 11A to 11Cand FIG. 12 can be manufactured.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 4

In this embodiment, an oxide semiconductor film that can be used for atransistor of one embodiment of the present invention is described.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “perpendicular” indicates that an angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly also includes the case where the angleis greater than or equal to 85° and less than or equal to 95°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described. Note that a CAAC-OS can be referredto as an oxide semiconductor including c-axis aligned nanocrystals(CANC).

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachlayer of metal atoms has a morphology reflecting a surface over whichthe CAAC-OS film is formed (hereinafter, a surface over which theCAAC-OS film is formed is referred to as a formation surface) or a topsurface of the CAAC-OS film, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan-view TEM image), metal atoms are arranged in a triangularor hexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

FIG. 14A is a cross-sectional TEM image of a CAAC-OS film. FIG. 14B is across-sectional TEM image obtained by enlarging the image of FIG. 14A.In FIG. 14B, atomic arrangement is highlighted for easy understanding.

FIG. 14C is local Fourier transform images of regions each surrounded bya circle (the diameter is about 4 nm) between A and O and between O andA′ in FIG. 14A. C-axis alignment can be observed in each region in FIG.14C. The c-axis direction between A and O is different from that betweenO and A′, which indicates that a grain in the region between A and O isdifferent from that between O and A′. In addition, the angle of thec-axis between A and O continuously and gradually changes, for example,14.3°, 16.6°, and 26.4°. Similarly, the angle of the c-axis between Oand A′ continuously changes, for example, −18.3°, −17.6°, and −15.9°.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) indicating alignment are shown. For example, whenelectron diffraction with an electron beam having a diameter of 1 nm ormore and 30 nm or less (such electron diffraction is also referred to asnanobeam electron diffraction) is performed on the top surface of theCAAC-OS film, spots are observed (see FIG. 15A).

From the results of the cross-sectional TEM image and the plan-view TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in theplan-view TEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears frequentlywhen 2θ is around 56°. This peak is derived from the (110) plane of theInGaZnO₄ crystal. Here, analysis (φ scan) is performed under conditionswhere the sample is rotated around a normal vector of a sample surfaceas an axis (φ axis) with 2θ fixed at around 56°. In the case where thesample is a single crystal oxide semiconductor film of InGaZnO₄, sixpeaks appear. The six peaks are derived from crystal planes equivalentto the (110) plane. On the other hand, in the case of a CAAC-OS film, apeak is not clearly observed even when φ scan is performed with 28 fixedat around 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where the shapeof the CAAC-OS film is changed by etching or the like, the c-axis mightnot be necessarily parallel to a normal vector of a formation surface ora normal vector of a top surface of the CAAC-OS film.

Distribution of c-axis aligned crystal parts in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added may be altered and the proportion of the c-axisaligned crystal parts in the CAAC-OS film might vary depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak may also be observed when 2θ is around36°, in addition to the peak at 2θ of around 31°. The peak at 2θ ofaround 36° indicates that a crystal having no c-axis alignment isincluded in part of the CAAC-OS film. It is preferable that in theCAAC-OS film, a peak appear when 2θ is around 31° and that a peak notappear when 2θ is around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Therefore, atransistor including the oxide semiconductor film rarely has negativethreshold voltage (is rarely normally on). The highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasa low density of defect states, and thus has few carrier traps.Accordingly, the transistor including the oxide semiconductor film haslittle variation in electrical characteristics and high reliability.Electric charge trapped by the carrier traps in the oxide semiconductorfilm takes a long time to be released and might behave like fixedelectric charge. Thus, the transistor including the oxide semiconductorfilm having high impurity concentration and a high density of defectstates has unstable electrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film will be described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor film in some cases. In mostcases, the size of a crystal part included in the microcrystalline oxidesemiconductor film is greater than or equal to 1 nm and less than orequal to 100 nm, or greater than or equal to 1 nm and less than or equalto 10 nm. A microcrystal with a size greater than or equal to 1 nm andless than or equal to 10 nm, or a size greater than or equal to 1 nm andless than or equal to 3 nm, is specifically referred to as nanocrystal(nc). An oxide semiconductor film including nanocrystal is referred toas an nc-OS (nanocrystalline oxide semiconductor) film. In an image ofthe nc-OS film which is obtained with the TEM, for example, a grainboundary is not clearly detected in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different crystal parts in thenc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the diameter of a crystal part, a peak indicating acrystal plane does not appear. Further, a halo pattern is shown in aselected-area electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter (e.g., 50 nm or larger)larger than the diameter of a crystal part. Meanwhile, spots are shownin a nanobeam electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a probe diameter close to or smaller thanthe size of a crystal part. Furthermore, in a nanobeam electrondiffraction pattern of the nc-OS film, regions with high luminance in acircular (ring) pattern are shown in some cases. Moreover, in a nanobeamelectron diffraction pattern of the nc-OS film, a plurality of spots areshown in a ring-like region in some cases (see FIG. 15B).

The nc-OS film is an oxide semiconductor film that has high regularityas compared with an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film.Therefore, the nc-OS film has a higher density of defect states than theCAAC-OS film.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

In the case where an oxide semiconductor film has a plurality ofstructures, the structures can be analyzed using nanobeam electrondiffraction in some cases.

FIG. 15C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 10, an optical system12 below the electron gun chamber 10, a sample chamber 14 below theoptical system 12, an optical system 16 below the sample chamber 14, anobservation chamber 20 below the optical system 16, a camera 18installed in the observation chamber 20, and a film chamber 22 below theobservation chamber 20. The camera 18 is provided to face toward theinside of the observation chamber 20. Note that the film chamber 22 isnot necessarily provided.

FIG. 15D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 15C. In thetransmission electron diffraction measurement apparatus, a substance 28which is positioned in the sample chamber 14 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 10 through the optical system 12. Electrons passing through thesubstance 28 are incident on a fluorescent plate 32 provided in theobservation chamber 20 through the optical system 16. On the fluorescentplate 32, a pattern corresponding to the intensity of the incidentelectrons appears, which allows measurement of a transmission electrondiffraction pattern.

The camera 18 is installed so as to face the fluorescent plate 32 andcan take an image of a pattern appearing on the fluorescent plate 32. Anangle formed by a straight line which passes through the center of alens of the camera 18 and the center of the fluorescent plate 32 and anupper surface of the fluorescent plate 32 is, for example, 15° or moreand 80° or less, 30° or more and 75° or less, or 45° or more and 70° orless. As the angle is reduced, distortion of the transmission electrondiffraction pattern taken by the camera 18 becomes larger. Note that ifthe angle is obtained in advance, the distortion of an obtainedtransmission electron diffraction pattern can be corrected. Note thatthe film chamber 22 may be provided with the camera 18. For example, thecamera 18 may be set in the film chamber 22 so as to be opposite to theincident direction of electrons 24. In this case, a transmissionelectron diffraction pattern with less distortion can be taken from therear surface of the fluorescent plate 32.

A holder for fixing the substance 28 that is a sample is provided in thesample chamber 14. The holder transmits electrons passing through thesubstance 28. The holder may have, for example, a function of moving thesubstance 28 in the direction of the X, Y, and Z axes. The movementfunction of the holder may have an accuracy of moving the substance inthe range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10 nm to 100nm, 50 nm to 500 nm, and 100 nm to 1 p.m. The range is preferablydetermined to be an optimal range for the structure of the substance 28.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above will be described.

For example, changes in the structure of a substance can be observed bychanging (scanning) the irradiation position of the electrons 24 thatare a nanobeam on the substance, as illustrated in FIG. 15D. At thistime, when the substance 28 is a CAAC-OS film, a diffraction patternshown in FIG. 15A is observed. When the substance 28 is an nc-OS film, adiffraction pattern shown in FIG. 15B is observed.

Even when the substance 28 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether a CAAC-OS film is favorable can be determinedby the proportion of a region where a diffraction pattern of a CAAC-OSfilm is observed in a predetermined area (also referred to as proportionof CAAC). In the case of a high-quality CAAC-OS film, for example, theproportion of CAAC is higher than or equal to 50%, preferably higherthan or equal to 80%, further preferably higher than or equal to 90%,still further preferably higher than or equal to 95%. Note that theproportion of a region where a diffraction pattern different from thatof a CAAC-OS film is observed is referred to as the proportion ofnon-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedjust after deposition (represented as “as-sputtered”) and a top surfaceof a sample including a CAAC-OS film subjected to heat treatment at 450°C. in an atmosphere containing oxygen. Here, the proportion of CAAC wasobtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/second and the obtaineddiffraction patterns were converted into still images every 0.5 seconds.Note that as an electron beam, a nanobeam with a probe diameter of 1 nmwas used. The above measurement was performed on six samples. Theproportion of CAAC was calculated using the average value of the sixsamples.

FIG. 16A shows the proportion of CAAC in each sample. The proportion ofCAAC of the CAAC-OS film obtained just after the deposition was 75.7%(the proportion of non-CAAC was 24.3%). The proportion of CAAC of theCAAC-OS film subjected to the heat treatment at 450° C. was 85.3% (theproportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained just after the deposition. That is, heattreatment at a high temperature (e.g., higher than or equal to 400° C.)reduces the proportion of non-CAAC (increases the proportion of CAAC).Furthermore, the above results also indicate that even when thetemperature of the heat treatment is lower than 500° C., the CAAC-OSfilm can have a high proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmare diffraction patterns similar to that of an nc-OS film. Furthermore,an amorphous oxide semiconductor film was not able to be observed in themeasurement region. Therefore, the above results suggest that the regionhaving a structure similar to that of an nc-OS film is rearranged by theheat treatment owing to the influence of the structure of the adjacentregion, whereby the region becomes CAAC.

FIGS. 16B and 16C are plan-view TEM images of the CAAC-OS film obtainedjust after the deposition and the CAAC-OS film subjected to the heattreatment at 450° C., respectively. Comparison between FIGS. 16B and 16Cshows that the CAAC-OS film subjected to the heat treatment at 450° C.has more uniform film quality. That is, the heat treatment at a hightemperature improves the film quality of the CAAC-OS film.

With such a measurement method, the structure of an oxide semiconductorfilm having a plurality of structures can be analyzed in some cases.

A CAAC-OS film can be deposited by a sputtering method using apolycrystalline oxide semiconductor sputtering target, for example.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 5

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention is described with reference todrawings.

[Cross-Sectional Structure]

FIG. 17A is a cross-sectional view of a semiconductor device of oneembodiment of the present invention. The semiconductor deviceillustrated in FIG. 17A includes a transistor 2200 containing a firstsemiconductor material in a lower portion and a transistor 2100containing a second semiconductor material in an upper portion. As thetransistor 2100, any of the transistors described in Embodiments 1 to 3can be used, and in FIG. 17A, an example in which the transistor 103 isused as the transistor 2100 is shown. A cross-sectional view of thetransistors in a channel length direction is on the left side of adashed-dotted line, and a cross-sectional view of the transistors in achannel width direction is on the right side of the dashed-dotted line.

The semiconductor device in the cross-sectional view has a structure inwhich the transistor 2100 is provided with a back gate; however, astructure without a back gate may be employed.

FIG. 17A illustrates a structure in which an intermediate layer 2210 hasconductivity. In the case where the intermediate layer 2210 has aninsulating property, as in FIG. 18, the transistor 2200 is connected tothe transistor 2100 with a wiring 2205 and a wiring 2209.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial other than an oxide semiconductor (examples of such asemiconductor material include silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, and anorganic semiconductor), and the second semiconductor material can be anoxide semiconductor. A transistor using a material other than an oxidesemiconductor, such as single crystal silicon, can operate at high speedeasily. In contrast, a transistor using an oxide semiconductor has lowoff-state current.

The transistor 2200 may be either an n-channel transistor or a p-channeltransistor, and an appropriate transistor may be used in accordance witha circuit. Furthermore, the specific structure of the semiconductordevice, such as the material or the structure used for the semiconductordevice, is not necessarily limited to those described here except forthe use of the transistor of one embodiment of the present inventionwhich uses an oxide semiconductor.

FIG. 17A illustrates a structure in which the transistor 2100 isprovided over the transistor 2200 with an insulating layer 2201 and aninsulating layer 2207 provided therebetween. A plurality of wirings 2202are provided between the transistor 2200 and the transistor 2100.Furthermore, wirings and electrodes provided over and under theinsulating layers are electrically connected to each other through aplurality of plugs 2203 embedded in the insulating layers. An insulatinglayer 2204 covering the transistor 2100, a wiring 2205 over theinsulating layer 2204, and a wiring 2206 formed by processing aconductive film that is also used for a pair of electrodes of thetransistor 2100 are provided.

The stack of the two kinds of transistors reduces the area occupied bythe circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is usedfor the transistor 2200 provided in a lower portion, hydrogen in aninsulating layer provided in the vicinity of the semiconductor layer ofthe transistor 2200 terminates dangling bonds of silicon in thesemiconductor layer of the transistor 2200; accordingly, the reliabilityof the transistor 2200 can be improved. Meanwhile, in the case where anoxide semiconductor is used for the transistor 2100 provided in an upperportion, hydrogen in an insulating layer provided in the vicinity of thesemiconductor layer of the transistor 2100 becomes a factor ofgenerating carriers in the oxide semiconductor; thus, the reliability ofthe transistor 2100 might be decreased. Therefore, in the case where thetransistor 2100 using an oxide semiconductor is provided over thetransistor 2200 using a silicon-based semiconductor material, it isparticularly effective that the insulating layer 2207 having a functionof preventing diffusion of hydrogen is provided between the transistors2100 and 2200. The insulating layer 2207 makes hydrogen remain in thelower portion, thereby improving the reliability of the transistor 2200.In addition, since the insulating layer 2207 suppresses diffusion ofhydrogen from the lower portion to the upper portion, the reliability ofthe transistor 2100 also can be improved.

The insulating layer 2207 can be, for example, formed using aluminumoxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttriumoxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, a blocking film 2208 (corresponding to the insulating layer180 in the transistors 101 to 103) having a function of preventingdiffusion of hydrogen is preferably formed over the transistor 2100 tocover the transistor 2100 including an oxide semiconductor layer. Forthe blocking film 2208, a material that is similar to that of theinsulating layer 2207 can be used, and in particular, an aluminum oxidefilm is preferably used. The aluminum oxide film has a high shielding(blocking) effect of preventing penetration of both oxygen andimpurities such as hydrogen and moisture. Thus, by using the aluminumoxide film as the blocking film 2208 covering the transistor 2100,release of oxygen from the oxide semiconductor layer included in thetransistor 2100 can be prevented and entry of water and hydrogen intothe oxide semiconductor layer can be prevented.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, thetransistor 2200 can be a fin-type transistor, a tri-gate transistor, orthe like. An example of a cross-sectional view in this case is shown inFIG. 17D. An insulating layer 2212 is provided over a semiconductorsubstrate 2211. The semiconductor substrate 2211 includes a projectingportion with a thin tip (also referred to a fin). Note that aninsulating layer may be provided over the projecting portion. Theinsulating layer functions as a mask for preventing the semiconductorsubstrate 2211 from being etched when the projecting portion is formed.The projecting portion does not necessarily have the thin tip; aprojecting portion with a cuboid-like projecting portion and aprojecting portion with a thick tip are permitted, for example. A gateinsulating film 2214 is provided over the projecting portion of thesemiconductor substrate 2211, and a gate electrode 2213 is provided overthe gate insulating film 2214. Source and drain regions 2215 are formedin the semiconductor substrate 2211. Note that here is shown an examplein which the semiconductor substrate 2211 includes the projectingportion; however, a semiconductor device of one embodiment of thepresent invention is not limited thereto. For example, a semiconductorregion having a projecting portion may be formed by processing an SOIsubstrate.

[Circuit Configuration Example]

In the above structure, electrodes of the transistor 2100 and thetransistor 2200 can be connected in a variety of ways; thus, a varietyof circuits can be formed. Examples of circuit configurations which canbe achieved by using a semiconductor device of one embodiment of thepresent invention are shown below.

[CMOS Circuit]

A circuit diagram in FIG. 17B shows a configuration of a so-called CMOScircuit in which the p-channel transistor 2200 and the n-channeltransistor 2100 are connected to each other in series and in which gatesof them are connected to each other.

[Analog Switch]

A circuit diagram in FIG. 17C shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. With such aconfiguration, the transistors can function as a so-called analogswitch.

[Memory Device Example]

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 19A to 19C.

The semiconductor device illustrated in FIG. 19A includes a transistor3200 using a first semiconductor material, a transistor 3300 using asecond semiconductor material, and a capacitor 3400. Note that any ofthe transistors described in Embodiments 1 to 3 can be used as thetransistor 3300.

FIG. 19B is a cross-sectional view of the semiconductor deviceillustrated in FIG. 19A. The semiconductor device in the cross-sectionalview has a structure in which the transistor 3300 is provided with aback gate; however, a structure without a back gate may be employed.

FIG. 19A illustrates a structure in which an intermediate layer 2210 hasconductivity. In the case where the intermediate layer 2210 has aninsulating property, as in FIG. 20, the transistor 2200 is connected tothe transistor 2100 with a wiring 3305.

The transistor 3300 is a transistor in which a channel is formed in asemiconductor layer including an oxide semiconductor. Since theoff-state current of the transistor 3300 is low, stored data can beretained for a long period. In other words, power consumption can besufficiently reduced because a semiconductor device in which refreshoperation is unnecessary or the frequency of refresh operation isextremely low can be provided.

In FIG. 19A, a first wiring 3001 is electrically connected to a sourceelectrode of the transistor 3200. A second wiring 3002 is electricallyconnected to a drain electrode of the transistor 3200. A third wiring3003 is electrically connected to one of a source electrode and a drainelectrode of the transistor 3300. A fourth wiring 3004 is electricallyconnected to a gate electrode of the transistor 3300. A gate electrodeof the transistor 3200 is electrically connected to the other of thesource electrode and the drain electrode of the transistor 3300 and oneelectrode of the capacitor 3400. A fifth wiring 3005 is electricallyconnected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 19A has a feature that the potential ofthe gate electrode of the transistor 3200 can be retained, and thusenables writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is supplied to the gate electrode ofthe transistor 3200 and the capacitor 3400. That is, a predeterminedcharge is supplied to the gate electrode of the transistor 3200(writing). Here, one of two kinds of charges providing differentpotential levels (hereinafter referred to as a low-level charge and ahigh-level charge) is supplied. After that, the potential of the fourthwiring 3004 is set to a potential at which the transistor 3300 is turnedoff, so that the transistor 3300 is turned off. Thus, the chargesupplied to the gate electrode of the transistor 3200 is held(retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the gate electrode of the transistor 3200 is retained for along time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the gate electrode of the transistor 3200.This is because in the case of using an n-channel transistor as thetransistor 3200, an apparent threshold voltage V_(th) _(_) _(H) at thetime when the high-level charge is given to the gate electrode of thetransistor 3200 is lower than an apparent threshold voltage V_(th) _(_)_(L) at the time when the low-level charge is given to the gateelectrode of the transistor 3200. Here, an apparent threshold voltagerefers to the potential of the fifth wiring 3005 which is needed to turnon the transistor 3200. Thus, the potential of the fifth wiring 3005 isset to a potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_)_(L), whereby charge supplied to the gate electrode of the transistor3200 can be determined. For example, in the case where the high-levelcharge is supplied to the gate electrode of the transistor 3200 inwriting and the potential of the fifth wiring 3005 is V₀ (>V_(th) _(_)_(H)), the transistor 3200 is turned on. In the case where the low-levelcharge is supplied to the gate electrode of the transistor 3200 inwriting, even when the potential of the fifth wiring 3005 is V₀ (<V_(th)_(_) _(L)), the transistor 3200 remains off. Thus, the data retained inthe gate electrode of the transistor 3200 can be read by determining thepotential of the second wiring 3002.

Note that in the case where memory cells are arrayed to be used, it isnecessary that only data of a desired memory cell be able to be read. Inthe case where such reading is not performed, the fifth wiring 3005 maybe supplied with a potential at which the transistor 3200 is turned offregardless of the state of the gate electrode, that is, a potentiallower than V_(th) _(_) _(H). Alternatively, the fifth wiring 3005 may besupplied with a potential at which the transistor 3200 is turned onregardless of the state of the gate electrode, that is, a potentialhigher than V_(th) _(_) _(L).

The semiconductor device illustrated in FIG. 19C is different from thesemiconductor device illustrated in FIG. 19A in that the transistor 3200is not provided. Also in this case, writing and retaining operation ofdata can be performed in a manner similar to the semiconductor deviceillustrated in FIG. 19A.

Next, reading of data is described. When the transistor 3300 is turnedon, the third wiring 3003 which is in a floating state and the capacitor3400 are electrically connected to each other, and the charge isredistributed between the third wiring 3003 and the capacitor 3400. As aresult, the potential of the third wiring 3003 is changed. The amount ofchange in potential of the third wiring 3003 varies depending on thepotential of the first terminal of the capacitor 3400 (or the chargeaccumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×J/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of thefirst terminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potentialof the third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor materialmay be used for a driver circuit for driving a memory cell, and atransistor including the second semiconductor material may be stackedover the driver circuit as the transistor 3300.

When including a transistor in which a channel formation region isformed using an oxide semiconductor and which has an extremely lowoff-state current, the semiconductor device described in this embodimentcan retain stored data for an extremely long time. In other words,refresh operation becomes unnecessary or the frequency of the refreshoperation can be extremely low, which leads to a sufficient reduction inpower consumption. Moreover, stored data can be retained for a long timeeven when power is not supplied (note that a potential is preferablyfixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. Unlike in a conventional nonvolatile memory,for example, it is not necessary to inject and extract electrons intoand from a floating gate; thus, a problem such as deterioration of agate insulating film is not caused. That is, the semiconductor device ofthe disclosed invention does not have a limit on the number of timesdata can be rewritten, which is a problem of a conventional nonvolatilememory, and the reliability thereof is drastically improved.Furthermore, data is written depending on the state of the transistor(on or off), whereby high-speed operation can be easily achieved.

Note that in this specification and the like, it might be possible forthose skilled in the art to constitute one embodiment of the inventioneven when portions to which all the terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In otherwords, one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where there are severalpossible portions to which a terminal can be connected, it is notnecessary to specify all the portions to which the terminal isconnected. Therefore, it might be possible to constitute one embodimentof the invention by specifying only portions to which some of terminalsof an active element (e.g., a transistor or a diode), a passive element(e.g., a capacitor or a resistor), or the like are connected.

Note that in this specification and the like, it might be possible forthose skilled in the art to specify the invention when at least theconnection portion of a circuit is specified. Alternatively, it might bepossible for those skilled in the art to specify the invention when atleast a function of a circuit is specified. In other words, when afunction of a circuit is specified, one embodiment of the presentinvention can be clear. Further, it can be determined that oneembodiment of the present invention whose function is specified isdisclosed in this specification and the like. Therefore, when aconnection portion of a circuit is specified, the circuit is disclosedas one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

Note that in this specification and the like, part of a diagram or atext described in one embodiment can be taken out to constitute oneembodiment of the invention. Thus, in the case where a diagram or a textrelated to a certain part is described, a content taken out from thediagram or the text of the certain part is also disclosed as oneembodiment of the invention and can constitute one embodiment of theinvention. Therefore, for example, part of a diagram or a text includingone or more of active elements (e.g., transistors or diodes), wirings,passive elements (e.g., capacitors or resistors), conductive layers,insulating layers, semiconductor layers, organic materials, inorganicmaterials, components, devices, operating methods, manufacturingmethods, or the like can be taken out to constitute one embodiment ofthe invention. For example, M circuit elements (e.g., transistors orcapacitors) (M is an integer) are picked up from a circuit diagram inwhich N circuit elements (e.g., transistors or capacitors) (N is aninteger, where M<N) are provided, whereby one embodiment of theinvention can be constituted. As another example, M layers (M is aninteger) are picked up from a cross-sectional view in which N layers (Nis an integer, where M<N) are provided, whereby one embodiment of theinvention can be constituted. As another example, M elements (M is aninteger) are picked up from a flow chart in which N elements (N is aninteger, where M<N) are provided, whereby one embodiment of theinvention can be constituted.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 6

In this embodiment, an RF tag that includes the transistor described inthe above embodiments or the memory device described in the aboveembodiment is described with reference to FIG. 21.

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system in which an object or the like isrecognized by reading the individual information, for example. Note thatthe RF tag is required to have extremely high reliability in order to beused for this purpose.

A configuration of the RF tag will be described with reference to FIG.21. FIG. 21 is a block diagram illustrating a configuration example ofan RF tag.

As shown in FIG. 21, an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A transistor having a rectifying functionincluded in the demodulation circuit 807 may be formed using a materialwhich enables a reverse current to be low enough, for example, an oxidesemiconductor. This can suppress the phenomenon of a rectifying functionbecoming weaker due to generation of a reverse current and preventsaturation of the output from the demodulation circuit. In other words,the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800 described in thisembodiment.

Next, the structure of each circuit will be described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage in the rectifier circuit 805.Note that a limiter circuit may be provided on an input side or anoutput side of the rectifier circuit 805. The limiter circuit controlselectric power so that electric power which is higher than or equal tocertain electric power is not input to a circuit in a later stage if theamplitude of the input alternating signal is high and an internalgeneration voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the memory circuit described in the above embodiment can be usedas the memory circuit 810. Since the memory circuit of one embodiment ofthe present invention can retain data even when not powered, the memorycircuit can be favorably used for an RF tag. Furthermore, the memorycircuit of one embodiment of the present invention needs power (voltage)needed for data writing significantly lower than that needed in aconventional nonvolatile memory; thus, it is possible to prevent adifference between the maximum communication range in data reading andthat in data writing. In addition, it is possible to suppressmalfunction or incorrect writing which is caused by power shortage indata writing.

Since the memory circuit of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 7

In this embodiment, a CPU that includes the memory device described inthe above embodiment is described.

FIG. 22 is a block diagram illustrating a configuration example of a CPUat least partly including any of the transistors described in the aboveembodiments as a component.

The CPU illustrated in FIG. 22 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and a ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 22 isjust an example in which the configuration is simplified, and an actualCPU may have a variety of configurations depending on the application.For example, the CPU may have the following configuration: a structureincluding the CPU illustrated in FIG. 22 or an arithmetic circuit isconsidered as one core; a plurality of the cores are included; and thecores operate in parallel. The number of bits that the CPU can processin an internal arithmetic circuit or in a data bus can be 8, 16, 32, or64, for example.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 22, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, any of thetransistors described in Embodiments 1 to 3 can be used.

In the CPU illustrated in FIG. 22, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data retaining by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data retaining by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

FIG. 23 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, a switch 1203, a switch 1204, a logic element 1206, acapacitor 1207, and a circuit 1220 having a selecting function. Thecircuit 1202 includes a capacitor 1208, a transistor 1209, and atransistor 1210. Note that the memory element 1200 may further includeanother element such as a diode, a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used asthe circuit 1202. When supply of a power supply voltage to the memoryelement 1200 is stopped, a ground potential (0 V) or a potential atwhich the transistor 1209 in the circuit 1202 is turned off continues tobe input to a gate of the transistor 1209. For example, a first gate ofthe transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily providedas long as the parasitic capacitance of the transistor, the wiring, orthe like is actively utilized.

A control signal WE is input to the first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

Note that the transistor 1209 in FIG. 23 has a structure with a secondgate (second gate electrode; back gate). The control signal WE can beinput to the first gate and the control signal WE2 can be input to thesecond gate. The control signal WE2 is a signal having a constantpotential. As the constant potential, for example, a ground potentialGND or a potential lower than a source potential of the transistor 1209is selected. The control signal WE2 is a potential signal forcontrolling the threshold voltage of the transistor 1209, and Icut ofthe transistor 1209 can be further reduced. The control signal WE2 maybe a signal having the same potential as that of the control signal WE.Note that as the transistor 1209, a transistor without a second gate maybe used.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 23illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 23, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 23, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a layer formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon layer or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor layer. Further alternatively, in the memory element 1200,a transistor in which a channel is formed in an oxide semiconductorlayer can be included besides the transistor 1209, and a transistor inwhich a channel is formed in a layer or the substrate 1190 including asemiconductor other than an oxide semiconductor can be used for the restof the transistors.

As the circuit 1201 in FIG. 23, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor layer is extremely low. For example, the off-statecurrent of a transistor in which a channel is formed in an oxidesemiconductor layer is significantly lower than that of a transistor inwhich a channel is formed in silicon having crystallinity. Thus, whenthe transistor is used as the transistor 1209, a signal held in thecapacitor 1208 is retained for a long time also in a period during whichthe power supply voltage is not supplied to the memory element 1200. Thememory element 1200 can accordingly retain the stored content (data)also in a period during which the supply of the power supply voltage isstopped.

Since the above-described memory element performs pre-charge operationwith the switch 1203 and the switch 1204, the time required for thecircuit 1201 to retain original data again after the supply of the powersupply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the signalretained by the capacitor 1208 can be converted into the onecorresponding to the state (the on state or the off state) of thetransistor 1210 to be read from the circuit 1202. Consequently, anoriginal signal can be accurately read even when a potentialcorresponding to the signal retained by the capacitor 1208 varies tosome degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Furthermore, shortly after thesupply of the power supply voltage is restarted, the memory device canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a digitalsignal processor (DSP), a custom LSI, or a programmable logic device(PLD), and a radio frequency identification (RF-ID).

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 8

In this embodiment, configuration examples of a display device using atransistor of one embodiment of the present invention are described.

Configuration Example

FIG. 24A is a top view of the display device of one embodiment of thepresent invention. FIG. 24B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display device of one embodiment of the presentinvention. FIG. 24C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance withEmbodiments 1 to 3. The transistor can be easily formed as an n-channeltransistor, and thus part of a driver circuit that can be formed usingan n-channel transistor can be formed over the same substrate as thetransistor of the pixel portion. With the use of any of the transistorsdescribed in the above embodiments for the pixel portion or the drivercircuit in this manner, a highly reliable display device can beprovided.

FIG. 24A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 701, a first scan line driver circuit702, a second scan line driver circuit 703, and a signal line drivercircuit 704 are formed over a substrate 700 of the display device. Inthe pixel portion 701, a plurality of signal lines extended from thesignal line driver circuit 704 are arranged and a plurality of scanlines extended from the first scan line driver circuit 702 and thesecond scan line driver circuit 703 are arranged. Note that pixels whichinclude display elements are provided in a matrix in respective regionswhere the scan lines and the signal lines intersect with each other. Thesubstrate 700 of the display device is connected to a timing controlcircuit (also referred to as a controller or a controller IC) through aconnection portion such as a flexible printed circuit (FPC).

In FIG. 24A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the substrate 700 where the pixel portion 701 is formed.Accordingly, the number of components which are provided outside, suchas a driver circuit, can be reduced, so that a reduction in cost can beachieved. Furthermore, if the driver circuit is provided outside thesubstrate 700, wirings would need to be extended and the number ofwiring connections would increase. When the driver circuit is providedover the substrate 700, the number of wiring connections can be reduced.Consequently, an improvement in reliability or yield can be achieved.One or more of the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 may bemounted on the substrate 700 or provided outside the substrate 700.

[Liquid Crystal Display Device]

FIG. 24B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A gate wiring 712 of a transistor 716 and a gate wiring 713 of atransistor 717 are separated so that different gate signals can besupplied thereto. In contrast, a data line 714 is shared by thetransistors 716 and 717. The transistor described in any of Embodiments1 to 3 can be used as appropriate as each of the transistors 716 and717. Thus, a highly reliable liquid crystal display device can beprovided.

The shapes of a first pixel electrode layer electrically connected tothe transistor 716 and a second pixel electrode layer electricallyconnected to the transistor 717 are described. The first pixel electrodelayer and the second pixel electrode layer are separated by a slit. Thefirst pixel electrode layer has a V shape and the second pixel electrodelayer is provided so as to surround the first pixel electrode layer.

A gate electrode of the transistor 716 is connected to the gate wiring712, and a gate electrode of the transistor 717 is connected to the gatewiring 713. When different gate signals are supplied to the gate wiring712 and the gate wiring 713, operation timings of the transistor 716 andthe transistor 717 can be varied. As a result, alignment of liquidcrystals can be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710,a gate insulating film functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode layer orthe second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat shown in FIG. 24B. For example, a switch, a resistor, a capacitor,a transistor, a sensor, a logic circuit, or the like may be added to thepixel circuit illustrated in FIG. 24B.

[Organic EL Display Device]

FIG. 24C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Owingto such a mechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

FIG. 24C illustrates an applicable example of a pixel circuit. Here, onepixel includes two n-channel transistors. Furthermore, digital timegrayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving are described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line formed over the same substrate as the commonelectrode 728.

As the switching transistor 721 and the driver transistor 722, thetransistor described in any of Embodiments 1 to 3 can be used asappropriate. In this manner, a highly reliable organic EL display devicecan be provided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least a forward thresholdvoltage.

Note that gate capacitance of the driver transistor 722 may be used as asubstitute for the capacitor 723, so that the capacitor 723 can beomitted.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, voltage higher than the voltage of the power supplyline 727 is applied to the gate electrode layer of the driver transistor722. Note that voltage higher than or equal to voltage which is the sumof power supply line voltage and the threshold voltage Vth of the drivertransistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage greaterthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage Vth of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order for the driver transistor722 to operate in a saturation region, the potential of the power supplyline 727 is set higher than the gate potential of the driver transistor722. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 724 in accordance with the videosignal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the presentinvention is not limited to that shown in FIG. 24C. For example, aswitch, a resistor, a capacitor, a sensor, a transistor, a logiccircuit, or the like may be added to the pixel circuit illustrated inFIG. 24C.

In the case where the transistor shown in any of the above embodimentsis used for the circuit shown in FIGS. 24A to 24C, the source electrode(the first electrode) is electrically connected to the low potentialside and the drain electrode (the second electrode) is electricallyconnected to the high potential side. Furthermore, the potential of thefirst gate electrode may be controlled by a control circuit or the likeand the potential described above as an example, e.g., a potential lowerthan the potential applied to the source electrode, may be input to thesecond gate electrode through a wiring that is not illustrated.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. Examples of a display element, a displaydevice, a light-emitting element, or a light-emitting device include anEL (electroluminescent) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor which emits light depending oncurrent), an electron emitter, a liquid crystal element, electronic ink,an electrophoretic element, a grating light valve (GLV), a plasmadisplay panel (PDP), a micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD)element, an electrowetting element, a piezoelectric ceramic display, ora carbon nanotube, which are display media whose contrast, luminance,reflectivity, transmittance, or the like is changed by electromagneticaction. Note that examples of display devices having EL elements includean EL display. Examples of display devices including electron emittersare a field emission display (FED) and an SED-type flat panel display(SED: surface-conduction electron-emitter display). Examples of displaydevices including liquid crystal elements include a liquid crystaldisplay (e.g., a transmissive liquid crystal display, a transflectiveliquid crystal display, a reflective liquid crystal display, adirect-view liquid crystal display, or a projection liquid crystaldisplay). Examples of display devices including electronic ink orelectrophoretic elements include electronic paper.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 9

In this embodiment, a display module using a semiconductor device of oneembodiment of the present invention is described with reference to FIG.25.

In a display module 8000 in FIG. 25, a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002. Note thatthe backlight unit 8007, the battery 8011, the touch panel 8004, and thelike are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for the display panel 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 so that an optical touch panel function is added.An electrode for a touch sensor may be provided in each pixel of thedisplay panel 8006 so that a capacitive touch panel function is added.

The backlight unit 8007 includes a light source 8008. The light source8008 may be provided at an end portion of the backlight unit 8007 and alight diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated fromthe printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. Note that the battery 8011 is not necessary in the casewhere a commercial power source is used.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 10

The semiconductor device of one embodiment of the present invention canbe used for display devices, personal computers, or image reproducingdevices provided with recording media (typically, devices whichreproduce the content of recording media such as digital versatile discs(DVDs) and have displays for displaying the reproduced images). Otherexamples of electronic devices that can be equipped with thesemiconductor device of one embodiment of the present invention aremobile phones, game machines including portable game consoles, portabledata appliances, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), and vending machines. FIGS.26A to 26F illustrate specific examples of these electronic devices.

FIG. 26A illustrates a portable game console including a housing 901, ahousing 902, a display portion 903, a display portion 904, a microphone905, a speaker 906, an operation key 907, a stylus 908, and the like.Although the portable game machine in FIG. 26A has the two displayportions 903 and 904, the number of display portions included in aportable game machine is not limited to this.

FIG. 26B illustrates a portable data terminal including a first housing911, a second housing 912, a first display portion 913, a second displayportion 914, a joint 915, an operation key 916, and the like. The firstdisplay portion 913 is provided in the first housing 911, and the seconddisplay portion 914 is provided in the second housing 912. The firsthousing 911 and the second housing 912 are connected to each other withthe joint 915, and the angle between the first housing 911 and thesecond housing 912 can be changed with the joint 915. An image on thefirst display portion 913 may be switched depending on the angle betweenthe first housing 911 and the second housing 912 at the joint 915. Adisplay device with a position input function may be used as at leastone of the first display portion 913 and the second display portion 914.Note that the position input function can be added by providing a touchpanel in a display device. Alternatively, the position input functioncan be added by provision of a photoelectric conversion element called aphotosensor in a pixel portion of a display device.

FIG. 26C illustrates a laptop personal computer, which includes ahousing 921, a display portion 922, a keyboard 923, a pointing device924, and the like.

FIG. 26D illustrates an electric refrigerator-freezer including ahousing 931, a door for a refrigerator 932, a door for a freezer 933,and the like.

FIG. 26E illustrates a video camera, which includes a first housing 941,a second housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. Images displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942.

FIG. 26F illustrates an ordinary vehicle including a car body 951,wheels 952, a dashboard 953, lights 954, and the like.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 11

In this embodiment, application examples of an RF tag of one embodimentof the present invention will be described with reference to FIGS. 27Ato 27F. The RF tag is widely used and can be provided for, for example,products such as bills, coins, securities, bearer bonds, documents(e.g., driver's licenses or resident's cards, see FIG. 27A), vehicles(e.g., bicycles, see FIG. 27B), packaging containers (e.g., wrappingpaper or bottles, see FIG. 27C), recording media (e.g., DVD or videotapes, see FIG. 27D), personal belongings (e.g., bags or glasses),foods, plants, animals, human bodies, clothing, household goods, medicalsupplies such as medicine and chemicals, and electronic devices (e.g.,liquid crystal display devices, EL display devices, television sets, orcellular phones), or tags on products (see FIGS. 27E and 27F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, the RF tag 4000 is fixed to each product by being embedded inpaper of a book, or embedded in an organic resin of a package. Since theRF tag 4000 of one embodiment of the present invention can be reduced insize, thickness, and weight, it can be fixed to a product withoutspoiling the design of the product. Furthermore, bills, coins,securities, bearer bonds, documents, or the like can have anidentification function by being provided with the RF tag 4000 of oneembodiment of the present invention, and the identification function canbe utilized to prevent counterfeiting. Moreover, the efficiency of asystem such as an inspection system can be improved by providing the RFtag of one embodiment of the present invention for packaging containers,recording media, personal belongings, foods, clothing, household goods,electronic devices, or the like. Vehicles can also have higher securityagainst theft or the like by being provided with the RF tag of oneembodiment of the present invention.

As described above, by using the RF tag of one embodiment of the presentinvention for each application described in this embodiment, power foroperation such as writing or reading of data can be reduced, whichresults in an increase in the maximum communication distance. Moreover,data can be held for an extremely long period even in the state wherepower is not supplied; thus, the RF tag can be preferably used forapplication in which data is not frequently written or read.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

This application is based on Japanese Patent Application serial no.2013-231209 filed with Japan Patent Office on Nov. 7, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: an oxideinsulating layer; a first intermediate layer and a second intermediatelayer over the oxide insulating layer, the first intermediate layer andthe second intermediate layer being apart from each other; a sourceelectrode layer over the first intermediate layer; a drain electrodelayer over the second intermediate layer; an oxide semiconductor layerthat is electrically connected to the source electrode layer and thedrain electrode layer and is in contact with the oxide insulating layer;a gate electrode layer overlapping with the oxide semiconductor layer;and a gate insulating film between the oxide semiconductor layer and thegate electrode layer.
 2. The semiconductor device according to claim 1,wherein the gate electrode layer overlaps with the source electrodelayer and the drain electrode layer.
 3. The semiconductor deviceaccording to claim 1, wherein each of the first and the secondintermediate layers is a single layer or stacked layers comprising atleast one material selected from the group consisting of aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, yttria-stabilizedzirconia, titanium nitride, tantalum nitride, gold, platinum, palladium,and ruthenium.
 4. The semiconductor device according to claim 1, whereinan area of a top surface of the first intermediate layer issubstantially the same as an area of a top surface of the sourceelectrode layer or is larger than the area of the top surface of thesource electrode layer, and wherein an area of a top surface of thesecond intermediate layer is substantially the same as an area of a topsurface of the drain electrode layer or is larger than the area of thetop surface of the drain electrode layer.
 5. The semiconductor deviceaccording to claim 1, wherein shapes of top surfaces of the gateelectrode layer, the gate insulating film, and the oxide semiconductorlayer are substantially the same as one another.
 6. The semiconductordevice according to claim 1, wherein the oxide insulating layercomprises oxygen at a higher proportion than that of a stoichiometriccomposition.
 7. The semiconductor device according to claim 1, whereinthe oxide semiconductor layer comprises In-M-Zn oxide, and wherein M isat least one of Al, Ti, Ga, Y, Zr, Sn, La, Ce, Nd, and Hf.
 8. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor layer includes crystals with c-axis alignment in adirection perpendicular to a surface of the oxide semiconductor layer.9. The semiconductor device according to claim 1, wherein the oxidesemiconductor layer has stacked layers including a first oxidesemiconductor layer, a second oxide semiconductor layer, and a thirdoxide semiconductor layer in this order from the oxide insulating layerside.
 10. The semiconductor device according to claim 1, wherein theoxide semiconductor layer includes stacked layers including a firstoxide semiconductor layer and a second oxide semiconductor layer in thisorder from the oxide insulating layer side and a third oxidesemiconductor layer covering the stacked layers.
 11. The semiconductordevice according to claim 9, wherein a bottom of a conduction band ofthe first oxide semiconductor layer and a bottom of a conduction band ofthe third oxide semiconductor layer are closer to a vacuum level than abottom of a conduction band of the second oxide semiconductor layer by0.05 eV or more and 2 eV or less.
 12. The semiconductor device accordingto claim 9, wherein the first oxide semiconductor layer, the secondoxide semiconductor layer, and the third oxide semiconductor layer eachinclude an In-M-Zn oxide, wherein M is at least one of Al, Ti, Ga, Y,Zr, Sn, La, Ce, Nd, and Hf, and wherein an atomic ratio of M to In ineach of the first oxide semiconductor layer and the third oxidesemiconductor layer is larger than an atomic ratio of M to In in thesecond oxide semiconductor layer.
 13. The semiconductor device accordingto claim 1, further comprising an electrode layer, wherein the oxideinsulating layer, the oxide semiconductor layer and the gate insulatingfilm are interposed between the gate electrode layer and the electrodelayer.
 14. The semiconductor device according to claim 9, wherein thethird oxide semiconductor layer is in contact with a side surface of thefirst oxide semiconductor layer and a side surface of the second oxidesemiconductor layer.